Like LJH says - softdrink / water bottles. The clear stuff, not the soft milky stuff. Look for a triangular "recyclable" imprint in the plastic of the bottle near the bottom, it should have a 1 in it and say PETE below. At least this coke bottle I'm looking at now does. PETE is used for softdrink bottles because its gas permeability is quite low, so the drinks don't go flat on the shelf and it's great for keeping the oxygen out of your developer.

I use butane lighter-refill to exclude the oxygen. You must get it ALL out or the colour dev will die, even refrigerated.