Dark Horse? I can see that. Also the VIS company (state side) is trying to keep a "clean" kid freindly line of manga in general. In otherwords, up to mid-teens. The stuff that is obviously for older readers seems to be picked up by DH mostly because DH has a rep (even a while ago) for being more of a gritty comic series type thing (funny considering DH is a spin off of DC). And currently only Image has really looked like a real contender for the type of stuff that DH does. Image however also only seems to be doing American comics.
Anyways, if you have a Borders or Barns & Nobles, heck, almost any major book seller near you, they should be willing to order it for you if you ask (usually at no extra charge).