Bill,  Providing a key to a restored musical box is an unnecessary,
but nice touch to the job (except on some larger boxes where the
lock holds the door shut).

The easiest way to get one is to send or take the lock to a locksmith
and ask them to make you a key.  Find an old business and they will
probably reach into a big box of keys and pull out one that works.  Or
you can go to a company like VanDykes Restorers.  They have individual
keys and sets of samples.

Many boxes have locks that come apart with two small screws.  If not,
one can usually pry them apart and reshape the little tabs for re-use.
A visit to the clock cleaning solution, followed by a trip to the
Evapo-rust bottle will remove any crud and rust.

Some locks need a key with a hole in the end, to center the key over
a pin in the lock.  The size of the tab at the end can be determined by
measuring certain internal dimensions of the lock itself.  A visit to
the local antique shop will usually produce a selection of keys that
generally fit the requirements.  That leaves you with a little filing
or work with a jeweler's saw to make the matching slots and notches
until the new key does the job.

Regards,
Craig Smith