I use a brand called SuperSafe. They make pages for 3 ring binders that have 2~3~4 pockets per page, as well as individual sleeves. One thing to look for is, as you mentioned, ACID FREE. SuperSafe makes a PVC product, which is good, but I prefer the Mylar or Polyester sheets they are more rigid (like a film negative). The Mylar is recommended more often for long-term storage but I believe either will do the job. You are looking at around 75¢ ~ 80¢ per sheet or about 35¢ for an individual sleeve (Not for a notebook). I have a partial box of both the soft type and the rigid type (3 pocket) if you just need a few. OR check with a local coin shop, prices will vary slightly but that shouldn’t matter when just buying a small quantity. I have been using SuperSafe for as long as I can remember, but other companies make fine products too, I have just stuck with SuperSafe for consistency in my pages. Long answer to a short question! LOL