Yes, there are a few places you can get visual upgrades for the exterior of your car. I believe ever part of the car from the windshield to the front bumper is the same as a 4-door 3series of the same year model.
There are several aftermarket rear bumpers and spoilers. Try:
www.bavauto.com www.racdyn.com www.wingswest.com I have a custom spoiler on my Ti, its a universal wings west wing that fits nicely. I have also seen a Supra style wing, and two different lip style spoilers that can fit the trunk.
Actually, just do a bit of a search on the forums here, you will find plenty of information on aftermarket parts.