Where to buy Saitama Railway museum tickets

We checked all convenience stores in Ginza and no one is selling. Website said we should purchase in advance? Help!

At 7-Eleven, Lawson or Ministop convenience stores. There are special machines at which you can purchase tickets (you cannot purchase them at the cashier), however, I think that most of these machine have only Japanese user interfaces.

Check a ticket buying guide like this one for Loppi (Lawson):
https://allabout-japan.com/en/article/2701/
I haven't purchased or printed tickets for ages, but it is not too hard. Last set was from a 7/11.

https://kakkon.net/railway-mu-advance-ticket/
https://koseiblog.com/tepppaku-oomiya-ticket/
the above links are specialized for how to buy railway museum ticket, but explained in Japanese.
it seems to be not so easy, a lot of steps.