Since you're asking this question at the MyBB Community forums, you're likely to get some rather biased answers favoring MyBB.
MyBB is, in my opinion, a far superior piece of software if all you're wanting is a forum system that's easy to get setup and just start using instantly. Installing plugins and themes is a breeze when compared to phpBB but it does lack some of the great extras that phpBB has available (especially in terms of themes).
As for good places to get premium themes, check out the following:
http://audentio.com
http://nanoskinners.com
http://dynaxel.com (currently shut unfortunately)
There are a few other theme design studios that are kind of up-and-coming but the above three are some of the best known ones.
MyBB is, in my opinion, a far superior piece of software if all you're wanting is a forum system that's easy to get setup and just start using instantly. Installing plugins and themes is a breeze when compared to phpBB but it does lack some of the great extras that phpBB has available (especially in terms of themes).
As for good places to get premium themes, check out the following:
http://audentio.com
http://nanoskinners.com
http://dynaxel.com (currently shut unfortunately)
There are a few other theme design studios that are kind of up-and-coming but the above three are some of the best known ones.