You can check out Van Dykes for forms, but I have to agree with what Dennis has said.....any taxidermist worth his salt should be able to alter/modifiy a form to fit what you want. I wanted my dog's first retrieve (drake mallard) mounted, but it had a large patch of feathers missing on the shoulder.....the taxidermist suggested a "preening" mount to help hide the defect, and it turned out very well.
As for forms, I think that the "side shoulder" mount (don't know what else to call 'em) makes a much nicer mount that just a straight on or slightly turned full shoulder mount...I also think (it may just be my imagination) that the more heavily turned 'side shoulder' mounts give you a little more wall space (at least that's how it appears with my pronghorn antelope).