Thank you both for these suggestions. I've been looking at the Light Impressions web site. They have a number of potential options, but their descriptions could be a little more clear in some cases.
There are some Mylar D sleeves (same thing as polyester?) and polypropylene sleeves available from LI. Most are sealed on three sides, and one, I believe, is sealed on two. From my research, I gather that polyester is considered slightly superior to polypropylene for long term storage.
The Pro-Line sleeves are polypropylene. It sounds as though they are available sealed on two or three sides.
I think the sleeves that are open on two or three sides that you then put into a paper envelope sound like the best way to protect the film without risking emulsion scratches from sliding in and out of a sleeve that is made like a pocket, sealed on three sides.
Glassine sleeves appear to be the most economical option, but I've seen a few recommendations to avoid them for storing film. I'm still researching this, though, as I really dislike plastic sleeves.