I've started to look for some as well. Here's where I've been looking:
Ram Air Restoration Performance Years Pontiacengines.net CI Ames
From what I can tell Ram Air says their manifolds are stock with a 2-1/2" outlet and all the others come from the same supplier. Don't know how true this is.
Would also like to know where the best place to get them from before I make the plunge.