I was thinking about buying a Nitro gas R/C car thing, and was wondering what do you use for fuel, and where do you get it?

First of all, it is either Nitro fuel OR Gas not both.

Nitro is used in a 2-4 stroke GLOW engine which uses a glow plug. You will need a Glow Plug igniter, once the nitro engine is running it will continue to do so, due to the glowing glow plug. (until you run out of nitro, no ooxygen or the muffler is plugged... given that the engine is properly tuned)

Gas is used in chain saw and weed whacker-engines, you can find these in FG, HPI Baja etc. RC vehicles. They don't have a low, and high mixture requirement. Way easier to tune and are more cost effective.

Nitro fuel can be found at most hobbyshops
Gas at your local gas station(regular vehicles)

Nitro fuel is made from oil and methanol mix just depends on what fuel you get.. I have a thinder tiger TS-4N on road nitro car its sick!