Five and Three volt ‘Buck’ Converters
These are small boards that take the full battery voltage and convert it down to 3v/5v for the receiver. These are available from a variety of sources, often several to a pack for quite reasonable costs.
Amazon – Buck Converters
Pololu also has a nice assortment of voltage down converters.
Servo Cables and Motor Controllers
Various sizes of servos, cables, connectors and wire are available from Pololu – Servo Cables and Connectors They also have crimpers, pins and connectors so you can make your own cables. I highly recommend getting a crimper and making your own cables, it makes for a tidy installation.
Pololu also has a nice selection of motor controllers (electronic speed controllers). Here is a link to their ESCs One of the nice things about these is that they have a selectable 3v/5v battery elimination circuit that will power either the PT Receiver or the AirRx.
Speakers
Parts Express has all sizes and quality of speakers that you can sort by size. Very handy for finding a speaker to fit in a tight location. Or, if you have the room, something that will do a really full range.
Batteries
For G Scale locomotives, 4 cell lipos- which give you 14.8 volts are a good choice. A really great place to get them from is Hobby King They have a web interface where you can search by size, which is real handy if you are trying to squeeze a control system and battery into a small space. They also have lipo chargers.
ALWAYS use a good quality charger made explicitly for Lipo batteries to charge them. I cannot stress this enough. Lipos have a ton of power, treat them with respect and always use a fuse in your installs.