They do not need to be thick. I recommend that you buy something that is 100% cotton, and relatively tightly fitted. You can even use men's undershirts.
Here are a few that I found with a quick search
Vermont Country Store
Buy.com
Never argue with an idiot. They always drag you down to their level, and then they beat you with experience. --Twain
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Surgery 2/10/93 A/P fusion T4-L3
Surgery 1/20/11 A/P fusion L2-sacrum w/pelvic fixation