I also recommend the Art Profolio by ITOYA. I use them for many stamp sheets and oversized covers, unfolded aerogrammes, even small posters and newspapers that I want to keep protected. Further, they are all acid free and ligin free, so they will easily protect stamp and other philatelic materials without worry. They are available in a whole range of sizes to accommodate anything from small business cards all the way up to "poster size" of 18" x 24":
http://www.artprofolio.com/Art_profolio.htmWhile there are certainly on-line sources for these items as suggested in an earlier post, most local chain-type hobby and craft stores carry them in inventory (i.e. Michael's, AC Moore, Hobby Lobby, etc.) and the key is to find the respective craft stores 40% or 50% coupon (usually available in their flyers or on their web site) which makes the product very affordable.