+1 for the local hobby shop, if possible. Once you find a good shop or two you can build up a rapport. Support them and they'll help support you!
The RC car hobby is a bit of a hot mess (hardly anything is packaged with dimensions or specs listed) and you'll need experienced advice plus the ability to compare your broken part with what they've got hanging on the wall. Sometimes even being able to bring in your whole car to do some comparison shopping is great. And with the right kind of person behind the counter it can be immensely fun!
Sometimes, however, you just can't find what you want on the peg board and you'll need to buy online (I'm thinking Ebay here for all the weird old stuff and "take-apart" bits). That's okay.
But definitely give your local shop the support. This hobby would kinda suck if it weren't for local shops. And I've have spent much more money on parts that wound up not fitting or working properly.