There are many places that you can find merch at! However not all will ship internationally. Here are a few pointers for buying from Japan!
To buy from stores that do not ship internationally, you will need to use a proxy (or middle-man).
Note that all of these will charge a fee for their services. Be sure to research which will work best for you!
There are more than this, and possibly there’s a store just for your series! Search around and see what you can find, and you can always request your proxy buy from it.
No Proxy Needed!
If you don’t want to go the route of using a proxy, here are a few sites that ship internationally!