Albertson's bakery is selling some of its bread in an unusually strong, tough, tear-resistant, and "crisp" bag. I could swear it feels just like my polycryo (cross-linked polyolefin) groundsheet, in bag form. Go to Albertson's and check out the 'English Toasting Bread.' In small print I notice it is "dist. by SUPERVALU Inc, Eden Prarie, MN."
EDIT: Capacity about 5L.
Maybe there are other/better sources for tough & light plastic bags?