You can buy direct from SPA Technique, Racer Parts Wholesale is a SPA dealer too. I dealt with RPW on my Stack ST700SR, nice people.
I called SPA directly a few months ago with some questions and they were very helpful.
http://www.spatechnique.com/
http://www.racerpartswholesale.com/
My only gripe with steering wheel displays is they're great if the wheel only turns 1/2 turn total or so (i.e. open wheel car); on a multiple-turn steering box on a street car they can be a bit uncomfortable and/or slippery if you're cranked over and grab the display area of the wheel. The curly-cord wrapping around the steering column leading to the wheel isn't very sightly either, but at this point it would be function over form. Just something you might want to consider.