I'd say its an even split three ways
- sessions and other live aural sources,
- tune books (in which category I include the Internet for abcs, NWCs, and images,
- and recordings, be that CD/MP3 or YouTube.

I'm a tunes player rather than a singer, so quite often I'll be looking for a particular type of tune to put to a dance (I'm currently auditioning 48-bar polkas for example). But also, a lot of the time, I'll just hear a tune somewhere and think 'I want to learn that', and will then acquire it from whichever of the three main categories of sources I've outlined above come up with the goods first!