Regarding cylinder music box keys, are you talking about a keywound
music box that needs a key to wind it or are you needing the key for
the case? There is a big difference as to which key: one means that
the box won't work, the other means that there is a nice finishing
touch to the box.
If you're talking about the power [winding] key, you can check with
Nancy Fratti -- she has a few spare from boxes that suffered a tragic
run and will never see the salon again. If you're talking about the
case lock key, you're best bet is to give it up and replace the lock
or cut a new key from steel.