This unprinted shirt is the same great model I use for my screenprinting work. It's Gildan 6.1oz, 100% preshrunk cotton, and is a little bulkier than the thin, smallish, and stretchy blue undershirt that's standard for ODU's.
I personally prefer this model to the issued 'undershirt', and always wore these under my own ODU's while on active duty. Super comfy, and you don't look like a you're in a muscle contest (which I'd lose) if you take off your ODU top.
These come in sets of three, for the sake of accounting & shipping efficiency.
Note: The short sleeve shirts are really popular nationwide. My local CVS sometimes has a bin of them for $3 each...you might want to look around locally to see if you can find them before buying here!