If you have the Witch Hut, make sure it is upgraded as high as possible. It is an outstanding orc producer. The evolving building from the upcoming Buried City event is also going to be an outstanding orc producer based on data from Beta.
Also, if you have a 1 time shortage of orcs then you can make some quickly by using time boosts on your armories. This is a temporary kind of solution you rarely use because it will eat so many time boosts. It is not a long term answer.