I've been wondering the same thing. You'll have to take measurements from your car and check ebay. That's what I've been doing, but it's so tough getting responses from the sellers regarding sizes.
Good luck. Derek had a thread about making one before. He constructed a rear strut bar with carbon fiber tube. Do a search and you may find it. Derek's pretty great at explaining stuff and answering questions. (sorry derek if i'm giving you more work to do, but I just can't help it. You've been great responding to my questions!).