I'm going to get a shirt from d-c.org to wear to Glendale. And I think I'm going to have "David-Cook.org" added onto it locally.
I've ordered from Cafepress before (not the DC shirts yet). The Tshirts are pretty much just like any other Tshirt. Read the descriptions carefully -- if they say "thin" (edit: or "lightweight") or such, those are the lighter cotton that some may prefer more than others. They usually have Made in the USA options, too.
Actually, their customer service is really good. I had a shirt arrive in the wrong color for some reason, and they just said, "Keep it. It's on us, and we're shipping the correct one out now."