I'm using DeLorme's Earthmate App on the iPad. That does what Gaia does, right?
Yes and no. Delorme can plot waypoints and create tracks. I believe you can import tracks as well but I have never done it. Gaia offers a lot more map options and other features. Earthmate maps for large areas are easy to download but I find downloading the forest service maps to a lot less convenient than Gaia.
Now for the original question. Finding tracks.
TrailsOffroad.com
FunTreks books and free downloads of waypoints for tracks they have written about.
GaiaGps.com and HemaExplorer but searching does not work well. More of a repository for tracks that people like me will store, share, and reference with others.
http://gpsxchange.com is good but you have to upload at least one track in order to be able to download something.
Look for trip reports here, Expedition Portal, ih8mud, etc where people share tracks along with their reports.
TrailDamage..com
Ok that should keep you busy. :)