Japan Direct is in your neck of the woods. Might try there (I think they do engines AND swap services.) I have no personal experience with them, however they seem like one of the more reputable places.
Check out
http://www.carolinanissans.com I believe DrifterJerry runs Japan Direct, and he's a frequent poster on the forum. I've spoken with him a couple of times and seemed knowledgable and friendly each time.
Good Luck...
(edit - Forgot,
http://www.usedengines.com is Japan Directs website, fyi.)