There are many izakaya(居酒屋) or kaiten-zushi(sushi) 回転寿司 restaurants
that serve local menu(郷土料理 kyodo ryori) of Hokkaido and northern Tohoku,
but the catch of "Gokko ごっこ/ゴッコ(Hotei-uo/ホテイウオ)" is not so much,
so there are few opportunities to eat in out of Hokkaido, including Tokyo.
So, it can only be displayed on irregular menus,
it is difficult if you do not ask the restaurants in advance.
Although it seems to hit sometimes when searching at Twitter or
as "ごっこ汁(-jiru)" or "ごっこ鍋(-nabe)", but very rare, can not expect it.
Female fishes(メス mesu) who have eggs(卵 tamago) are popular and expensive.https://www.zukan-bouz.com/syu/%E3%83%9B%E3%83%86%E3%82%A4%E3%82%A6%E3...
(seems about 500Y/fillet, more than 1500Y/one whole fish)
If you stay the accommodation with a kitchen, it would be easier to find and buy yourself and cook it.
Even without going to the Tsukiji fish market, (largest in Japan)
there is a possibility that you can buy it at a big supermarket if lucky.
(Since it was introduced by a famous TV program, awareness is already increasing)
And, basically ごっこ鍋(nabe) is a kind of "collagen pot(コラーゲン鍋)",
so it taste close to anger(アンコウ/ankou), suppon turtle(スッポン), or chicken wings(手羽/teba) based,
but you can find eating places very easy to eat and no seasonal.