Where can I get a new key for an old lock?
For a lock that's 50-60 years old, I'd be tempted to replace the whole thing. Just go to the hardware store and pick a similar one up so all you need to do is unscrew the old and screw in the new. Typically the inside of the lock is screwed into the outside half of the lock, and the bolt is screwed into the door on the side.

The biggest gotcha is if the hole changes size or position, not uncommon with an old lock, which is why you're better off finding a very close match. When I had to enlarge a hole in a metal door, I managed to rig two hole saws together on a common center mount so that the smaller hole saw guided the larger saw. Without this particular hole saw model, or if you need to shift the position of the hole, you'd need to first patch the hole so the saw has something to keep it from sliding all over the door, and of course this only works if you're enlarging the hole to include the entire space of the old lock.