Paying 1 large scale for each.
Your group doesn't need them nearly as much as I do.
The suggested method is to zerg off while they're buffing, kill the first flayer and loot the stone. They'll never even notice one is missing. Alternatively, after everyone sets off head in the opposite direction to the outside group and find a flayer that way... after getting a stone say, "sorry guys, every other group I've run with moves clockwise" and rejoin the main group.
Let me know when you have some stones for delivery.
Also now looking for vials of lunar water.