I got my tube substitution book from Radio Shack years ago but it is no longer available. Parts Express sells a tube substitution handbook:
http://www.partsexpress.com/pe/pshowdetl.cfm?&DID=7&Partnumber=500-090
http://www.partsexpress.com/pe/pshowdetl.cfm?&DID=7&Partnumber=500-090