I think the song you are talking about might be Peggy-O. Personally one of my favorite Dead songs.
Check out
http://www.archive.org. It's a free P2P music sharing website. There are tons of full live shows. You'll find tons of Dead, Ratdog, Widespread, ect.
Some are very good sound quality, and others are not. Some of the shows are rated, which can give you an idea of the ones with the better sound quality.