I have listed Several websites that may be able to help you.
Since I bought a house built in 1924, I have the same problem. I have ALL orignal doorknobs & hardware but have 2 or 3 doors that the locking mechanism is not working or broken.
Some places I have located the OLD harware is at Antique shops, OLD hardware stores & garage sales. Since I live in PA there are MANY of these up here. I was successful in referbishing one of the 3 locks.
You can try the web. There are ALL sorts of web sites that will sell the hardware, complete doorknob sets & some seel the mechanisms. Some will period date them & give you an idea of what period they came from.