Available at onemanga.
Note that onemanga does include scans of other material (like, say, Naruto, Bleach, One Piece, and gazillions of other manga) with disputable legality, so advance at your own risk.
But still HOLY CRAP A METROD MANGA (squees then faints)
Edit: An even BETTER place.
Edit #2: These guys are translating Metoid from the beginning.
Looks like the fifth volume/book isn't finished being translated.
Nevertheless, a nice find. It features characters familiar to those who read the old Super Metroid comic as well as played the Fusion game. Although some of the data seems to contradict certain things established in other places, it's still a good read.
Yeah, I've heard about this emanga on Wikipedia and Wikitroid (a wiki dedicated soley to Metroid) according to these sources this manga is considered the canon backstory of Samus by many.