Casterracingusa.com, trick-parts.com, if you have a lhs, they could special order them in no problem.
the car would be the same as any other, except you are running 2) 2s packs instead of one huge 4s one. Usually racers run 2 4000 packs or 5000 packs, but it depends on how long you need to run. The packs need to be series wired to get up to 14.8 volts.
Casters are also a lot easier to do 6s setups with 1300kv motors, etc... Same speed, same runtime, but a lot cooler temps and easier on electronics. Check your track rules.