McMaster certainly sells FDA approved beverage tubing, and I think a few people (I know Yooper's one of them) have tubing from McMaster on their systems. So I'd look for anything FDA approved, I think you can sort on their website to only look at beverage tubing. I don't think wall thickness matters too much for beverage tubing. You could do something like this if you want 1/8" wall thickness, or you could definitely do 1/16" thickness no problem:
But: I'll say that you might not save any money buying everything on McMaster vs. a full kit from KegConnection or some place similar, and I highly recommend barrier tubing (Bev Seal Accuflex) which isn't available there. If you're set on McMaster, get everything you can there (gas tubing, regulators, valves, etc.). You'll still need keg couplers or QDs, faucets, etc. so you'll need to go to a LHBS or place an order from a kegging supply company, you can just grab the bev. tubing there if you want the good stuff.