There are several sources of commercial 117V phase couplers for the X10 Signals:
(Smarthome has an extensive listing) but there are other sources with unique plug in solutions:

Wire in:
ACT Coupler Repeater

Passive (No signal amplification, or store and forward repeating)
Plug in:
Clothes Dryer Plug in coupler

Wire in:
X-10 Pro Passive Phase Coupler

Homebuilt capacitive phase coupler:

The commercial solution is the safest route for most people, and with the plug in models available for under $25 it is difficult to understand why even the highly skilled electrical type would make their own. It wasn't always this way and several X10 pioneers have been successful with homebuilt phase couples but those that do take that approach are experienced working with power line voltages and understand the risk that they are taking. The following section contains information from group posts from those individuals, many of which are experienced electrical engineers, technicians, and others with electrical safety experience such as HAM Radio Operators. Although there is a very simple "capacitor across the two power line phases" solution to couple the X10 signal it is very important to understand the safety implications and ensure that the solution is completed in an electrical code compliant way for your area. (In summary - if you read the following and say to youself - that seems easy, I have no idea why it works but I'll give it a try, you are better advised to purchase a commercial solution!)

Comments on home built phase couplers - (long).
1) "Most" homes don't need an repeater so passive phase couplers are sufficient.
But how to tell ... It is more likely by the complexity of the wiring than the square footage - I've paid attention to star configuring my X10 paths (mentioned in another post) so they radiate out from the breaker panel, I inject Misterhouse at the panel plug, and passively couple on a 240V box nearby - it covers 4000 square feet with approximately 30 X10 devices. Since I've gone away from anything but mh sending signals (e.g. no collisions) I would estimate the reliability around 99%. I have gotten rid of several marginal modules (likely 5) over the last few years.

In a smaller house - about 1200 sq.ft. I had the X-10 source (one of those clocks back in 1989) at the end of the almost longest circuit - constant grief until I moved it to the basement near the breaker panel (and the center of the star). That being said - a repeater will definitely work in all situations - but does increase the chance of message collisions (each command gets sent 3 times).
2) Technically there are two options for passive circuits(commerical or home built).
a) Capacitive & inductive "high frequency short" between the two hot conductors
(across approximately 240V).
b) A transformer coupled signal transfer circuit. Like the circuits within the modules that have tuned inductor-capacitor circuits - then an isolation transformer - and then a tuned inductor-capacitor injection circuit.
3) When you make your own (likely just a capacitor) you need to consider safety in both normal and failure modes.
a) Normal operation Voltage - 240V AC across the phases is more like 300V peak-to-peak.
- A good rule of safety is at least a factor of 2 times so a component value of 600V (just like Tom's below).
- Some capacitors are polarized - that is they only like DC voltage applied one way.(Don't use those for AC) Most ceramic, and polystyrene capacitors are not polarized - they also tend to survive infrequent voltage spikes. You should have a non- polarized capacitor. If the capacitor is specified for VAC even better.
b) Normal operation current - For AC signals - capacitors sort of act like resistors - its called reactance.
Our 0.1 uF capacitor acts like a 26,500 Ohm resistor(at 60Hz) and a 13 Ohm resistor at (120Khz - X10 carrier frequency) which is why this circuit works for us. But there still is a 60 Hz current - something like 9 mA "leakage between the two hot phases - two concerns emerge - power heating, and shock hazard. The 0.1 uF capacitor will dissipate about 2VA ~ 2W of heat which could built up over time - so make sure it has a wayout of where you put the capacitor.
c) Cross phase leakage current - The second concern is safety
- Make sure that the capacitor is only applied where both breakers are either on - or off (ganged) since 9mA of leakage current could still kill you.
For example
- lets say that the capacitor was placed across two breakers that were not ganged
- one breaker has been turned off while someone changes a receptacle
- the other breaker is still engaged
- a light plugged into the receptacle would not light which usually means it is safe to proceed. The wires touched together would not spark - safe? Connect the wire with the leakage current to one hand of the repair man and the other hand to ground - 9mA through the heart -enough for V-fib and a 911 call. (The commercial solution with the isolation transformer does not have that safety risk.)
d) Capacitor Failure Mode - open (of all failures this is most likely).
Your x-10 network no longer has the phase coupler - no sparks, etc. Just confusion.
e) Capacitor Failure Mode - short circuit. Usually the result of over voltage punching through between the capacitor plates. You now have a direct short across 240V with peak currents of 30 to 50 A (yes even on a 15 Amp breaker). The capacitor is most likely just going to explode and issue a bit of smoke as it self fuses - end of story. (And it becomes the failure mode above ...)
f) Capacitor Failure Mode - resistive short circuit. Similar to above but the capacitor heats and then arcs as the metal starts to burn - it sprays metal around which causes other arcs - tends just to burn out if there is nothing else to catch on fire.
Overall - a good quality capacitor with the correct voltage rating has a reliability nearly as high as wires. I've used a home built capacitive phase coupler in three different homes for 17 years - never had one fail. I like using the capacitors designed for surge protectors because they are designed to withstand voltage spikes induced from lighting strikes, and the insulation on their plates and leads is robust.
(In other electronic endeavors I've seen lots of capacitor failures - in almost all cases it is over-voltage through either surges, or bad designs - that kill them, in rare cases physical damage).
Building your own Phase Coupler and Noise Blocker

An idea for a plug in version for a split phase kitchen plug: (This was written in response to a home renter without an electric dryer. i.e. Could not modify the house wiring or use the dryer plug in version).

Happily there is usually another place in your home that the two phases of the 120V appear if you have a reasonably modern home (>1979 in Canada I think) - the kitchen should have split plugs (e.g. each outlet on a dual socket in the kitchen has it's own 15 amp supply - but both must be tripped off with the same breaker action - which is easiest to accomplish by routing a 3 wire 2 phase cable to the box and connecting one phase to each outlet). You should have a "double breaker" like the stove breaker but only 15 amps if this is the case and if you are lucky it's marked as kitchen outlets. It is then possible to use a outlet expander plugged into on of those split plugs to house the capacitor and inductor of a passive phase coupler - it can also be easily removed if you move. Such as:Canadian Tire Product# 52-1568-0 but make sure it is one that you can take apart and put back together (some are moulded sealed). Even better is to modify a surge protector since it will be easier to solder to inside the box (because you would replace the surge suppressor items with phase couplers - and if you get lucky it will have the capacitor you need)