After buying numerous library cookbooks with their nice protective covers I decided to figure out how to do the same for my unprotected ones.
Searching lead me to Brodart one maker of these jackets. They can be ordered for correct width and length. But only a library would need so many of each specific size. The alternative is to order a roll and cut to length. These rolls also come in widths of which I've found the 12" works for most cookbooks; Brodart part number 10-342-001. This method requires wrapping the clear portion over the inside of the jack on one edge, so that one size can be used for 12 or less.
You'll also need tape. I've been using filament tape.
Share a roll with someone if it's too much for you or start an association or club to do this.