Wow! Antique-shop.com forums are in the big #1 spot on Google for the search term "antique shop forum", for 4 years running!Thanks to YOU.
Your house is big enough to make a perfect setting for the cupboard, which regardless of age or origin will be the only one in the neighborhood I bet. It reminds me almost of a vestry from a church?Here's a seller from overseas that carries interesting pieces: http://www.bae.be/inventory/individual-items
Go to a few antique shops in your area or look on ebey !! Will not be difficult to find a skeleton key that will work !! Don`t discount the "not so big" keys !! You just need one that will turn the tumbler !!
used to import boatload containers full of items like this in the 70's until the 90's
Can you take a picture of the inside? and back?