The Postal Service - Recycled Air. It's original quite electronic but there's an acoustic version that is simply amazing.
Iron and Wine - Such Great Heights. Originally a Postal Service song (or was it Death Cab?) but it's beautiful when played acoustically.
PM me if you want the songs on mp3.
Edit: Also, by Aussie band Sodastream, a song called Welcome Throw. I doubt you'll find it on any P2P network, so PM me and I can send it. There's a bit of cello on it but I bet you could do well on guitar alone. Check out
http://www.sodastream.net.au/ and go to the Music section, you can see a few videos and listen to some MP3s from there. I'm not a huge fan of their videos but check out the songs 'Heaven on the Ground', 'Welcome Throw' and 'Varkhala'.