Hi Rue-
I assume you are interested in commercial, rather than free public domain titles. There are sources for those like project Gutenberg (sp?), etc., but if you want newer and popular titles, it's a little different.
I'm fairly new to ebooks, myself. I've bought a couple at fictionwise.com, when they were on sale.
In addition, I have found that Amazon.com's prices on ebooks are pretty good. Also,
http://www.simonsays.com/content/index.cfm?sid=358 is a good and fairly cheap source for some Simon and Shuster books, if that's who published what you want. I like non-fiction and history, and they've had good prices on some stuff I like.
You might want to check out
www.mobileread.com which is a good ebook-focused site.
--- Andrew