We are a costume company and have thousands of sewing patterns which never fit back into their original envelope. We use Staples 10" x 13" kraft envelopes to store the patterns and keep them neatly organized. They keep their sturdy shape don't curl or mildew are easy to store and sort.
When using these type of envelopes always tape the bottom of the envelope just in case the seal is not good. I haven't had any problems with Staples envelopes though.