JayT's link looks pretty good. I would also check out these sites to compare fees.
Celga, Inc. - Your source for Japanese goods.
Shopping Mall Japan - Japanese Auction Bidding and website order service!
Rinkya - Japan Deputy Bidding Service Fees
Personally, I've tried SMJ and Celga in the past and had no problems, but I think no matter which choice you go, it's kind of a big money sink and don't expect to get your item too quickly unless you want to pay 10x your tshirt's worth for shipping.