There's a prototype Mega Man 5 cartridge currently up for sale over at the Nintendo Age forums. With no specific price, all you have to do is make a (realistic) offer and it's yours.
Now, because this is a Mega Man 5 proto and not a Rockman 5 proto, bear in mind there won't be any major differences in graphics or gameplay; it's likely a sample cartridge that was sent out to the media for preview/review purposes or testing purposes, too. Nevertheless, it should be a nice addition to anyone's Mega Man collection
Full details and more pics here.