Inquisitor

[Lysander]

Now that I've reached a higher level, I am able to find Inquisitors for sale at the Furious Paladin's House in Marshan Swamp. They weren't available to me until now. (There might be more elsewhere, not sure.) Nevertheless, upgrading Priests is the way to go until you reach higher levels, I guess.

[MAXIMVS]

I've found it worth going back from time to time to check if different troops are for sale (or emptied stock replenished!).

After you have leveled up occasionally (at key points in the story) there are different troops available.