If you go to Takayama, there's a place at this address:
日本岐阜県高山市上一之町2. Mizuma
https://tabelog.com/en/gifu/A2104/A210401/21012620/
That sells it.
I went to a friends house some years ago and we made a version of kiritampo for Northern (Akita) style nabe.
My daughter was very young at the time so I was delighted to learn that there was a style of Mochi that wasn't so potentially lethal to babies and small children. From that introduction I did a bit of research and I've been making Gohei Mochi ever since.
This personal interest means that I'll buy it in Japan whenever I see it, the sweet miso sauces (often with walnuts) vary enormously, so I'm not sure I've tried the definitive version yet.
My memory isn't the best, but I've had it near Takao San Guchi station, up in the Fukushima region near Mt Bandai and at a couple of festivals in Tokyo itself.
If you have a Kitchen Aid with a paddle setting you can make it yourself out of standard cooked Japanese rice (a suribachi and wet pestle takes rather longer). The sauce is pretty easy, mostly trial and error. Perfect food for an autumn BBQ.