I've had some luck with Robert Johnson lyrics at the following site

xroads.virginia.edu/~MUSIC/rjhome.html

It is not complete, but has lyrics to many of his songs.

Another one that you might try is the following, I find it an excellent site for blues lyrics,

www.geocities.com/BourbonStreet/Delta/2541/blrjohns.htm

Good luck,

Steve