In my version of STW WE it is possible to retrain any unit anywhere, even if that province cannot produce this specific unit. This may be a bug or overlooked, but it means that you can build your Ninjas elsewhere and retrain them in your armoury province to pump them up.
If it doesn't work in your version, you have to specialise your metal rich provinces. One for cavalry, one for infantry, one for special units like Kensai, gunpowder or battlefield Ninjas etc. There are enough such provinces to do this.