Choosing a proper power source for for basic RC car with 4 motors
2S LiPo with 5V UBEC is what you want.  Hit your favourite RC model seller for those.  I use HobbyKing (as the links show).

The capacity choice of the LiPo depends on


How much power your motors etc need, and
How long you want it to run for


You could also go for 3S or 4S to get more power if your chosen UBEC supports the voltage.