Home Depot should have it, it's used in ceiling lighting fixtures but at my local store it's kept in with the sheets of plywood, drywall, and other large flat things.

I'm new to false bottoms, but just made my first today (over LECA) using nylon screen mesh found on windows. Black landscaping fabric also seems to be popular. I don't know if one is better (landscaping fabric has smaller 'holes'), but you may have an easier time finding an appropriate sized piece of window screening. Mine was recycled from an old window.