I only use the following sites myself because they're the only ones I know are trusted, but they should have plenty of apps for you to choose from I would think...
Blackberry App World (app available; typically installed on most BB devices)
http://appworld.blackberry.com/webstore/category/0?lang=en
Crackberry Mobile App Store (app available)
http://software.crackberry.com/platformMainMobile.asp?platform=5