you probably wont find an off the shelf HO alt for the xterra beyond what you have already found. the mean green is one of the only ones i was aware of.
there are companies that make custom alternators tho. ohio generator, iraggi, alterstart, stinger, etc. but they arent cheap. it would be easier to find a GM 1 wire alternator and switch to that. they can be found up to 300 amp pretty easily. you would have to fab up your own bracket tho.
and a little tip, call them. all of these companies can do a lot more than what is on their website. talking to people will get better results than sending emails and clicking links.