There are multiple threads for this.
This is how you search for them:
Go to google
Search for Scion FRS/ Subaru BRZ exhaust
Look through any link you can possibly find
If you want to get more specific, search "site:ft86club.com exhaust"
What I did, was I went to ft86speedfactory.com found a couple exhausts I liked the look of, listened to sound clips, made a short list of the ones I wanted then looked for reviews of them. However, without a budget, without knowing how loud you want your exhaust, without knowing the kind of style (straight pipe, muffled, single exit, dual exit, etc.) it's kind of hard to help you narrow it down.
I've had 3 exhausts on the car. OEM (looked nice, but too quiet), cheap eBay one (Spec-D Tuning, was overall happy with it, but I wanted something in a single exit and louder) and now I have a Tomei Expreme Ti 60R which I am beyond pleased with.
This is a freebie, although @
Koa may not be so kind.