items that sell by 20s restock at double if you buy them, eventually they will peak at 999 scrolls, but it takes around 3 hours to get them to that level.
I get mine in Skara Brae, especially if that happens to be the cheap faction town at the time. 2 mages, 2 real estate agents in the carpenter's, 2 more in the town hall and one of the map makers at the shipwright's. Not sure why only one of those sells scrolls.

Gives you a total of 140 scrolls at the start, building to 8991 when you've worked them up.