Gohei Mochi is a regional delicacy found at Central Mountain area of Japan: Hida, Takayama area is known for Gohei Mochi but also can be found at part of Nagano, Toyama, and Yamanashi. Usually flavored with soy sauce or miso based sauce.
Gohai Mochi is not totally unknown outside of the area known for Gohei Mochi, but same time it not one of top delicacies in Japan. Outside of the area known for Gohei Mochi, like in Tokyo and Nagoya, it is not like you will find Gohei Mochi on every other location. At Tokyo you have came across a place which had Gohei Mochi, but at Tokyo I think you simply have to know the place which has Gohei Mochi.
Since Gohei Mochi is not one of top delicacies in Japan, I do not think place which carry Gohei Mochi will put that information on a website or such. Japanese restaurant search/review website Taberogu only list two locations in Tokyo which possibly has Gohei Mochi. It does not mean only two locations has it in Tokyo, but likely many place do not bother to list Gohei Mochi as one of their offerings.
If you read Japanese, you can order Gohei Mochi from Internet and delivered to your hotel while you are visiting Japan.