You can check your local library website. I can download mobipocket and adobe ebooks from my library website (In addition to audiobooks, which is what I use it for most) current bestsellers even. I wish they had them formated for the Sony Reader, but those I get free from the library work fine on the BlackBerry.
Check here to see if your library participates:
http://search.overdrive.com/
Or just check your library website. If your local library doesn't participate you could get a library card at one that does. I know you can get an out of area card from NYC public library for 100.00 a year, and honestly they have the best selection. 100.00 for all the audiobooks I can listen to in a year is a pretty good deal for me.