I've been having a very happy experience buying my exhaust from @
Redline360, so I recommend you check out their excellent summary article on a variety of BRZ/FRS exhausts.
Since it was written in 2012, the article doesn't list ALL of the available exhausts, but it does break down what you get for MANY of the available exhaust types:
Subaru BRZ Exhaust and Scion FR-S Exhaust – Which is the best