Nope, just finding them is the trick. They move around from town to town. Usually you find them near the town sigil, but sometimes they are in an odd place. And sometimes the prices are great, and sometimes they are sky-high.
The scrolls will not actually be sold by the faction vendors, though. But in a town with faction vendors, the npcs at the mage shops will have comparable price reductions. So you can get necro regs and scrolls and even stuff like blank runes at good prices in the shops even though the faction vendors actually only sell magery regs.
Incidentally, faction vendors are good for lumber, bottles and ore, too.
And in general, in shops or with the faction guys, buying on weekends is good for this sort of thing ... chances are that more people are running up the same vendors, and that helps build volumes and keep them up. Sometimes you will find it hard to get there when stuff is in stock ... in such cases, I wait til a little later in the day when traffic slows down a tad but before volumes devolve again.