I improvised and used some pretty thick ( I think 4 mil, but I can go look if you think you'll try this ) black poly that I bought in huge rolls for covering parts of my garden in winter. I folded it over so that I cut two sheets at a time and cut out a big rectangle, sized so that when I folded it in half it was the size of the "envelope" I wanted. Everything is two sheets thick. Then I folded over the sides and taped with black duct tape. The top folds entirely over ( 4 thick ) and tapes to the front. I made three of these and use them to carry around my unexposed paper negatives for changing in my changing bag. I'm still using one stack that I made in July and there is no fogging on the paper, even though it has been in direct sunlight many many times. The roll itself was not all that expensive, maybe $20 ( 100 feet long and about 5 feet wide ), and the amount I used for the envelopes probably didn't cost more than a few cents. Works great and I'll make more if I need more paper storage bags. The only downside I've noticed is that the outside of the bag seems to get static and attract dust. I find myself wiping it off often, but none has gotten inside as far as I'm aware.