Many more-established antique shops usually have a group of old keys
for sale, usually for a couple of dollars each.  The key you need is
about 1-1/2 inches long and probably has a female end shank with
a quarter-inch or less square lock lever protrusion.  These locks are
generally a one-lever lock -- one piece of metal, spring-actuated when
the key projection lifts it to unlock the locking mechanism.

The antique malls generally do not handle these loose keys and an
established dealer usually has a box for old cabinets that need keys.

Happy hunting!
Gregory Filardo