I believe the stock at the orc castle "replenishes" after the Kordar mission, you have to retake the castle, not sure if you took it earlier than that?
Go back after Kordar and beat idiot over the head again (If you didn't do it before it's infested with Beholders). He will offer random orc troops but a good chance of getting catapaults. I know they're random at other locations but I've never met orcs anywhere else - love the randomness in this game!
BTW why have catapaults when you can have shamans and Evil Beholders
