You can try Hanes website (onehanesplace, I believe). They sell Champion clothes in every size imaginable, plus the normal Hanes t-shirts (I buy the long sleeve tees on their site).
WARNING TO MARRIED GUYS!!!: If you order from Hanes, be forwarned that most of their clothes are womens, with an emphasis on women's undies. Either you're wife will give you dirty looks when the catalog comes in the mail and starts thinking you're a pervert, or she'll run up a huge credit card tab ordering stuff from them. I take no responsiblity for this. YOU HAVE BEEN WARNED!!!