When deciding upon an archival sleeve, your choice material should be odorless, soft, and pliable. While Acetate and Mylar can appear similar, note that Mylar won't tear easily like Acetate.The addition of plasticizers nullifies a material's value as an archival medium.
Chemically stable material; it is transparent and boasts a high tensile strength. More expensive than others, but the best. Trade names are Mylar® and Melinex® by DuPont.
Most popular choice, polyethylene is a chemically stable, highly flexible, transparent or translucent plastic.
PVC is not as chemically stable as some other plastics, since it can emit hydrochloric acid as it deteriorates; you will notice a definite chemical odor. Don't use this stuff in your collection.
Where to Buy?
Bags Unlimited : limited site - get a catalog
Archivalware : you can request a catalog if they're not online yet...
Sleevetown : website feels good