I agree the adventure shop found at adventuregamers.com has a large variety of games to download. I have done it several times and I have found that it is easy, and the support staff readily available.
Also, if you play the hidden object games and have an account at Big Fish, they are getting more and more adventure games (found in the big file section). If you are a member, they can be the cheapest around.
Finally, Kheops has a bunch of downloadable adventure games. Again, I have found them to be very easy to work with, and I love their games.
Hope that helps!
Dyl'smom