The US mail-order retailer
Freestyle sells Foma, both as such as under one of its many house brands, "Arista.EDU Ultra" (the "Ultra" part is important; "Arista.EDU" is Forte -- or was, before Freestyle switched suppliers). Their house-brand prices are pretty low, at least by US standards. I don't know what shipping to Japan would cost or how exchange rates and relative product prices would affect things, but you might look into the possibility of ordering the house brand from Freestyle. If you prefer the OEM brand, I believe that
J&C (again in the US) is less expensive. J&C also sells some other Eastern European brands, such as Efke. It might be cheaper to order from a European supplier, but I don't have URLs handy.