@heero I know this thread is old but it still shows up 1st page when I search bbPress themes. So I thought it would be worth providing an upto date answer for future viewers.
The 1st stop I would recommend is checking out wordpress.org/themes where you can find thousands of themes.
However, if you don’t want to spend all day looking through their directory then check out Softwarefindr a platform were forum owners using bbPress give a shortlist of their recommended bbPress themes.