One website that I can recommend for you is Naipeszaca.com. They are located in Spain and sell a whole bunch of different kinds of decks, including some vintage ones. They do have international shipping, and in small packages it isn't too costly, however I have found that the price goes up a lot if you order a big pile of decks from them.
If you are into custom decks, I can recommend an old French printer called Grimaud. They did a lot of custom decks in 70's and 80's and some of them are really cool looking, like decks called 1900 or Jules Verne. Their decks can still be found, both opened and unopened and the prices seem to move a lot, as some sell them for 20-50 USD per deck, but if you can find a French seller, they might sell them a whole lot cheaper, about 10 USD per deck, or less. Grimaud is nowadays part of Cartamundi.
I come from Finland and I can tell you that there are no custom decks in here, except this one: viewtopic.php?f=8&t=8682&p=103853&hilit=Turku#p103853
If you want one, you can PM me.
Also I came about this seller just yestarday: https://www.rubylane.com/shop/twoforhisheels/ilist
There are lot of old decks on sale there, but the prices are quite high. I have no experience with this seller, but seems trustworthy.