Alastor flashes by in the tavern often; you can get it from him
I joined 2 (or even 3) stacks of horses (1 on the way to the left, where the Inferno outpost is, a second by the 2nd Castle, where the necrodragons are), that's almost 20 + I think brought about 12-15 from the dwell + built some in both Castles around week 3; by the end of month 2, there were about a hundred. I only built level 7s in one Castle and at the very end, didn't even buy them out in the end. Imho, they aren't needed, especially since the AI always hits them when it runs out of mana, and then there's an elf with Death Strike, plus an orc with Krag's special...
Finished in 4.5 months (stats), could be faster, I ran through all the upgrades. Dispel fell from the Temple in the humans' lands; bought Divine Wrath, Berserk, and Resurrection in the library