Protective plastic bag for shipping electronics

I'm in the process of selling off one of my integrated amps (it measures about 19" x 15" x 5") and though I have the original double box, I don't have the protective plastic bag that would have originally come with it, and I don't want to ship the amp without it. I've looked around pretty extensively on the internet to find a source for one of these bags (probably 3 or 4 mil thick) but no luck so far. Manufacturers must get them from SOMEWHERE. Does anybody know of a source? Thanks!