Metroid II and Super Metroid have also been re-released, but their packages are basically the same as the originals, with only minor changes. Again, the games are identical. A new color version of Metroid II is also supposed to be in the works, for use on the GameBoy Color system. But fundamentally, there are only three Metroid games. Check E-Bay if you're desperate, but be warned because prices can get way out of hand there.
Original copies are easy to come by in used game shops or online as well.
Try calling Nintendo's customer service line and ask them if they still have some available. Or try some used game shops. It's been out of print for quite a while, and I think The Place ran out when I featured it on the MDb and there was a rush on the item. So unless you or a friend live in Japan and have access to a used CD shop there, you're probably out of luck. When I find out, you'll find all the information I know right here on the MDb.