CafePress do (here). Other than that I'm not sure. The thing to look for would probably be the American Apparel "natural" t-shirt (part of their "Sustainable Edition"). I think it's their only organic shirt, but I'm not sure. At a guess though I'd say AA is the largest blank shirt producer to be using organic cotton for anything (though probably not the largest producer of organic blanks), so they're generally easier to find.
I wouldn't be surprised if other fullfillment services offer that same shirt, but I'm not sure.
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