I would say that since you just basically want the shell, even a cheap clearsoft P90 would suit your needs well. Give it a few coats of black Krylon, a couple coats of clear, and you've got a perfectly useable shell.
http://buyairsoft.ca/catalog/product...roducts_id=135
They're out of stock at the moment. Not sure who else carries it.
An airsoft P90 will more than likely cost you several hundred if you buy it here on the forum, unless you can get really lucky and find a Well P90(exactly the same gun as the clear model only in black). And expect to pay at least $200 and maybe more.