I've found the best place to farm these are Dracnids and Shiliskins in the Southwest (village area) quadrant of Stoneroot Falls at about 4x more drops then the other zones I checked out.
Dirty Vellum sells for 0 plat 9 gold 5 silver 2 copper when offered to Maree Rasumus spell research vendor near the small bank by level 51 human necromancer with 156 cha.... Current bazaar prices range from 985 to 1000 plat on bertox server.... Used to craft vellum (spell research trivial 243)along with celestial cleanser (player crafted only) and vellum solution (736 plat)...