If so, where can I find them? And by "custom" I mean avatars made with the in-client avatar editor specifically.

The avatar code is simply just a string of characters that the client processes and uses different body parts for. They are all stored on the server but you can get them locally. Custom avatars are not in any other format.

Alright, thanks. And just to be clear these are all the avatars that are in the Avatar Gallery?

Every single avatar.