Some locksmiths specialise in finding or replacing antique furniture keys. An armoire's locked door need not be a barrier to its owner. A locksmith with a suitcase full of supplies and key blanks can make a key to unlock a furniture piece. After first trying old keys, he may find it necessary to fabricate a replacement. Very often, the same skeleton key will unlock numerous pieces of furniture.
Find a skeleton key at an antique or thrift store. See if this will unlock the armoire. There are different styles to the end tabs of these keys so collect different shapes.
Contact a locksmith who knows how to make replacement keys for antique furniture. Locksmiths are listed in phone directories or online.
Check out the online sources for antique skeleton keys and purchase some with the possibility that one will open your armoire. Dealers may be able to hone in on your furniture style and year of manufacture to distinguish the type of key needed to unlock it.