You can find tunes all over the internet, both from ABC notation sites (more of this on the ABC info page), and other places such as FolkTuneFinder where, if you don’t know the name of a tune, you can put the notes on a stave to get some suggestions (although this can be rather hit-and-miss). If you are not a music reader, then there is a terrific resource of sound files for popular session tunes at the Lewes Folk Club site.
Paul Hardy has compiled a superb tunebook that is available as a free download, or you can pay a modest sum for a printed copy. When I last looked it contained over 600 tunes so should keep you busy for a while.
Tunebook Live! – more tunes than you can shake a stick at (when I last checked it was 12,317). Formats include dots, ABC, midi and PDF.
Hardcore English, published by EFDSS, has 300 popular tunes – some with alternative versions – and interesting info about their sources and first known published dates. It’s also ringbound, so stays open flat on a music stand! There is a companion CD with a selection of tunes performed by various artists.
A mega-tome which is not exactly something you would want to carry about with you, is Mike Ravens’ 1000 English Country Dance Tunes. This includes some facsimile pages from Playford manuscripts and earlier versions of tunes which you won’t find in other books. Pricey but properly bound so it won’t fall apart – probably an ideal candidate for a Christmas or birthday wish list.