I was actually looking for that, too, a little while back (win7 ultimate keys) and found that you can buy it online! But you either buy technet keys, which aren't quite what you want (priced $5-$50) or you can buy the real deal, but that is $200+
It's incredible, it is as if microsoft wouldn't want us to use win7 anymore! No way!
p.s. don't bother asking the seller if theirs is a genuine key, of course all of them are!