Sorry forgot to mention that keys often go missing because the lock is broken, and the key does'nt get used...a locksmith would soon tell you if they are still serviceable.
The locks should still be readily available from an old style hardware store...and as internal doors only have 2 or 3 levers in the lock. They tend to be fairly cheap if they need to be replaced (giving you new keys).... to identify the size of your lock, measure from the front edge to the back edge of the case... ( in inches).