The items/spells even units that are sold by vendors are completely RANDOM, there is no guarantee that you will be able to buy some rune mages later. In my first run I didn’t have maidens on the first 4 islands.
There are few vendors that selling humans after the *event*
, 3 in Greenland (2 if you killed baros), 1 on elven island and I think I saw somewhere later one or two.
However you can always increase them by using sacrifice spell.