A good place to check is:
search.overdrive.com
You can search for books/authors etc. or look for a library in your area and then see what they have to offer.
Actually, I've done searches for specific titles I'm interested in and sometimes an ePub result pops up with a few libraries listed...(check out "Long Lost" by Harlan Coben). There are a few libraries listed as having this in ePub format. Clicking on some of the libraries brings you to an error page though. I'm assuming that this means these libraries have purchased these titles in ePub format, but haven't yet added them to their website.
I'm still impatiently waiting for availability at one of the libraries I have a card for though.
dordale