Not a stockpage, but I can recommend these as a great way to store oversized philatelic material. Of course, I am not sure of the availability of them other than in the US/Canada. However, they are available in an array of sizes and offer similar protection as stock book pages would.
http://www.artprofolio.com/Art_profolio.htmJust bought an IA-12-8 last week with a retail price of $10, for 50% off (with store coupon) at $5, at an A.C. Moore Craft Store. Often stocked by the larger craft/art supply/camera stores.