I'm sure there are a lot of options out there. One that works for me is the "ITOYA Art Profolio". Here's one in a 4" x 6" size. But they have them as big as 18" x 24", too, so it is possible to use them for unfolded sheets, aerogrammes, booklets, postcards, covers, small posters, etc. Further they are archival safe and 100% acid free.

Although they can get a bit pricey (especially the larger sized ones), I typically buy them at a craft store such as Michael's or AC Moore (in the art departments) and if you shop the papers or internet you can often find a 40% or 50% off coupon, making them quite reasonable.
Note they lie flat, too, for scanning, such as in this example:
