Where do you suggest to look? I've been seeing quite a few that say "*Fastback Only". I think even the Cobb Tuning setup says that. The limitation may be on the larger downpipe though? I'm definitely a novice. Just got my car last week.
Okay I did some sleuthing on the interwebs and found out the S550 Mustang 'Vert's have an extra support brace to help stiffen the chassis. I posted a picture below for reference. Looks like some
exhaust systems will not clear the brace. I also saw where some owners had modified the brace in order to provide additional clearance. Heck, one guy used a hydraulic press and just bent the brace enough so it would clear the pipes.
So you got two choices. Purchase a system that specifically states it will fit the convertible model, or buy whatever system you want and be prepared to make some modifications.