You can export and import GarageBand projects in the OS X (Mac) version of the app. You can export projects from iOS to the OS X version. However there is no way to import a project file to the iOS version.
A lot of Google searching turned up a couple of online tutorials with project files for downloading, but I did not see any general repository of shared project files. I'm sure some must exist, but apparently they are not popular enough for them to show up, or more likely I'm just not using the best search terms.
On the other hand, GarageBand on the Mac is supposed to be able to import MIDI files. Not that this helps on the iPad in any way.