Looking for a store online that sells antique push knob parts (interior mechanisms)?

I'm trying to find interior door parts (inside the door mechanism) for an exterior door from the 1930's. The handle is intact but is missing a part on the inside which springs the push knob up and down. I'm only finding door handles but not interior mechanical replacements. I'm also looking for an explaination or diagram of how these old push button locksets work.