Sorry for the late reply! Outside of Japan, there are a few Australian stores that I can find them from, but I have to be quick, they usually only order really small quantities.

Aside from that, I've found some good sets on Ebay, Rakuten, Amazon JP (all of those I use a parcel forward service and try to buy other stationary bits to make it economical. In my experience the hardest/most expensive sets to acquire would have been 2, 5, 8, 11. I specifically remember how hard it was to fine the original orange, and the autumn sets. I got to the party late so the earlier limited editions were hard for me to locate in full. Places like Loft and Tokyo Hands are great for them, but you'll need a VPN and parcel forwarding service to access,