Hi,
It's unlikely to find such parts as... parts.
One of the methods to cut prices on toys is, to use the very cheapest solutions to everything, so there are no servos in those cheapies, just a motor moving the steering all the way left or right and the motor control has, at most, 2 speeds forward and 1 back.
You won't find quality gears in those either, everything is plastic, usually of a really weak kind and as much as possible is built into the chassis.
Differentials is mostly found in the more expensive toys (like around $100 and up, at least in Denmark, prices are probably lower where you are) and is substantially lower quality than if you buy an R/C part, but R/C parts has to withstand up to several hundred Watt - with some motors up to around 2 HP (around 1500W) and much heavier vehicles than a $20 toy car, so it's not a big surprise (sadly, cause there are some really fine stuff made for R/C cars).
Your best bet is to buy a cheap car (perhaps second hand) if you think you can use some of its parts, but if it's gears and such, take a look at eg. Vex Robotics, they have a good range of plastic gears and loads of other stuff.
