I saw a commercial for these this morning while in Tokyo and I happened to be heading to Sendai later that day. The first post office I went to I didn’t see anything so I thought maybe the photo I saw wasn’t actually stamps. Then I asked the tourist info desk at Sendai Station. Someone there heard someone got the stamps and Sendai Central Post Office earlier that day, so I walked there and sure enough, I saw signs for the special stamp sets. I went inside and there were quite a few people in the queue (I was 474 and they were on number 431). Eventually to help speed up the line they tried to have one clerk soley sell the stamp set, there was a little bit of an uproar as at first people thought they were going out of number but they didn’t. It seems many of those waiting were indeed there to buy Yuzuru’s stamp set (I saw a lot of people getting it).
End of story, I got the stamps

It was 1400 yen, I would’ve gotten Shoma too since these are pretty cool, but I didn’t have enough actual cash on me for my full purchase. So I’m not sure how widely available these are but they don’t appear to be offered at every post office.