The only time they
may be replenished is after you complete a certain (not just any) quest with the seller. If you can't find any more of your favourite creature I suggest using the sacrifice spell on a unit with the magic shield spell/effect on it (courtesy of the archmage unit) and then resurrect in the next round. That's how I increased the number of my elf stack as the only (!) stack of elves for sale anywhere had long since been bought.
