This article is from the Bagpipe FAQ, by Denise McNickle email@example.com with numerous contributions by others.
A partial list of available tune books might easily triple the length of
this document. Suffice it to say that there is a rather vast array of tune
books available for all types of pipe.
As to finding a specific tune, you're in luck. James Stewart maintains a
very thorough annotated index of tunes in very accessible format. This index
includes over 20,000 well-known and little-known tunes, and is freely
available over the Internet. See "Internet Resources", below, for downloading