Amazon.com has some older adventure games available for download for $1.25-$3.00 via their
Digital Games Summer Sale.
Syberia 1-2, Black Mirror 1-3, Secret Files 1-2, Nibiru Age of Secrets, Still Life, Scratches, Post Mortem, Return to Mysterious Island, some of the earlier Nancy Drew games, and lots of other stuff I'm not familiar with.