| If you want access to an almost unlimited library, just download a midi player (e.g. sweet midi for ipad). You can then google search for midi tracks of any songs you are interested in and use the midi tracks as backing. As its midi, you can control each instrument e.g. turn off the bass track, turn up the drums etc.
Songsterr is also good as an online resource. Haven't tried wikiloops - will have to give that a go! | 
