Check this site every day:
http://www.shopgoodwill.com/default.asp
Click on musical instruments.
I've seen everything from Tubas, to pianos, to Martins and Gibsons. Some nice, some total garbage. It's a potpouri, so you have to keep checking. I've bought about 30 instruments from the site, mostly guitars. Just got a beater banjo today. I never go very high, because the instruments keep coming and there is no end. I've probably bid on 500 or so of them over the last year.