I'm usign the black plastic stuff from TSC, it comes on a big roll. Although I don't drink the water in the garage, it's the same stuff that comes from the well into the house.
Check the local code, but I suspect you would be fine with either PVC, copper, or pex. I have never buried pex but if I were to go that route, I think I would want to sleeve it in PVC. I don't know about the black poly.