Best of course is always to make it yourself. But that's easy for me to say, being a tracker. And since I don't have time to make everybody a custom tune...

Check out e.g. United Trackers at http://www.united-trackers.org/ where you can search their music database. Just search for style "game" or "demo" and you'll find a bunch.

Another place is World of Game mods at http://www.mirsoft.info/gamemods.php. Though I'm not such a fan of using other games' music.

Chiptunes are nice as well, find them at e.g. http://www.chiptune.com/. However, the Galaxy sound engine (used in JJ2) SUCKS at playing chiptunes properly. >_<