Chris Cornell - can't change me
Sevendust - angels son
radiohead - street spirit (fade out)
SOme of these songs contain vocals, some contain small amounts of electric guitar....the first lot are pure accoustic instrumentals. Also i'd recommend very highly alot of Ennio Morricone songs from the spaghetti westerns he did the soundtrack for....some good stuff there ie a fistfull of dollars, for a few dollars more etc.
Obviously theres heaps more but this might give you something to consider at least.