I did scan real papers from old books and work on them with photoshop (using several layers with different blending modes and paint it soflty with stained brushes). I was very satisfied of the result. You just have to put them together and erase with a very soft brush (or with a low opacity) - grunge brush do very well for that) to avoid repetitive patterns. Oh, and don't have links in mind for now, I didn't really search on the internet now, since I collected a pretty good textures librabry on my PC over the years