You can try this link:
http://www.sctuners.com/bodykits_lam...c_firebird.htm
I've also seen a few other sites that can custom make the vertical door setup for your car, but they're all pretty expensive for what you get in my opinion. There are also do it yourself CDs that supposedly tell you how to do it.
To me, it seems like a lot of cash just to make a set of doors go upward rather than outward, but it's your car.
Hope it helps