Tinkerbell can be found in Never Land Jungle. On the second playthrough, she will appear near the set of Pete Smashers are moving to and fro. After painting her in, Mickey will explain to her what's going on. She'll happily return to the Fortress as long as Peter Pan is there, and take up a spot in his room.
Tinker Bell will also lend you a bit of pixie dust later, so that you can use the You Can Fly sketch.