When I get around to doing something with mine, I'm just going to fab one up. I have a slab of 1/2" stainless plate at home for the flange.
These guys are pretty reasonable for components:
http://www.verociousmotorsports.com/
If you want to go with something exotic like inconel, some of the Indy guys use Burns Stainless but the price is equally exotic:
www.burnsstainless.com