Reginary 10 Pcs 19x25 Inch Art Portfolio Bag Bulk: Clear Waterproof Portfolios Posters Storage Supply Bags with Zipper and Handle for Students, Teachers, Artists, Posters, Bulletin Boards(Blue)

I use these from Amazon for each class: 19x25 Portfolio Bags (this may not be the exact ones I bought as the purchase was as few years ago, but same size/material) They're really durable and fit full size paper from the large 18x24 watercolor and Bristol pads. They are waterproof and can be wiped clean if any paint, chalk, etc gets on them. I probably have a set of about 25 I use for different things and haven't had to buy new ones in years.