The North Georgia QRP Club will be making a commercial group-buy of NOGA T-shirts, in the near future. Until then, it's possible to home-brew your own NOGA T-shirt.
If you've never made your own T-shirts before, several manufacturers (HP, Canon, etc.) make T-shirt transfer material. It ususally comes in packs of 10 or 20 sheets, and is available at most office supply stores (Office Depot, Office Max, etc.). To make a T-shirt, print a reverse image onto the transfer material (this requires software which can flip Horizontal or Mirror the image).
To make things easier, we've already reversed the image.
The most popular color T-shirt so far has been an olive-drab or gray (in keeping with the concept of to-the-field operations).