BuddyPress.org

Nowhere? The only good themes out there right now are..

https://buddypress.org/extend/themes/

http://3oneseven.com/wpmu-buddypress-themes/

http://www.buddydress.com/category/buddypress-themes/ (badly supported)

http://premium.wpmudev.org/themes/ (paid)

Most people are still trying to work out there own themes, let alone work on other themes already. I myself am planning on doing a few themes in the near future as well.