kevin, i just looked at your site. very, very nice work. loud hawaiian shirts eh? well....you live a lot closer to there than i do.
stores like Joann Fabrics, or any other comprehensive fabric store has lots to choose from. for my dark cloth, which is quite light-tight, we used two (and that's the key to making fabrics light-tight) layers of a high thread count cotton. on sale at the time (not uncommon), the stuff was quite reasonable. such a store is a good place to start in any case.