Where can I replacement skeleton keys that work on doors that use this type of key?
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There are hundreds upon thousands of possible shapes for skeleton keys, in fact, they are more complex than the flat keys we use today. The chances of ever finding the right one is slim to none.
Take the lock to a locksmith and they can custom make a key to fit your lock.
A "skeleton' key is actually a very specific style of warded key.
By cutting away as much of the key as possible ('skeletonized") it misses the wards that are intended to make the key shape distinctive and it can then fit and operate multiple locks.
Local hardware stores often have skeleton keys that match the ones in use in the area.
You still might need to buy a few to find one that works, or learn how to modify one to get it to work.
A decent locksmith can make a warded key that will operate, though many will try to sell you a new (modern) lock.