Does anyone know of an online source of japanese books and manga that is based in the U.S., so i don't have to pay international shipping?

http://www.kinokuniya.com/us/ is the best I've found.

Depends what you're looking for, but you might be able to get them dirt cheap used at amazon or whatever and then forward with Tenso: http://www.tenso.com/ . Not really what you asked for, but whatever.