Try
TSC Apparel Look at the Tultex 0202. It is a 100% combed ringspun cotton. I use it all the time. Also the Anvil 980 is a nice shirt as well. It's slightly bigger than the Tultex, but still 100% combed ringspun, which is what alot of the shirts like the one you have up are. It's thin, 100% and really comfortable to wear. Also, you can get it wholesale for around $2 -$3 per piece.
Let me warn you though, their website is a little tricky to get on sometimes....