I use the LARGE plactic zip bags from the dollar store
15 x 15 and 18 x 22
Come to think of it .. I have a box that I use for my patterns that I have had for 54 years. It has the date stamped on it from the Post Office when it was mailed to me. Ha Ha some of the patterns are about that old too. I worked at a 5 and dime store in the fabric department and when they had a close out on the patterns, I grabbed a few. Some I have never used at all.
Syl