Copy and paste this into google maps: 甲府市 風俗店
There are a number of Soaplands in Kofu (much more than conservative Shizuoka) but don't be discouraged if you are rejected by a some of them. I have visited Kofu as a tourist before, but I haven't checked out any of their adult entertainment shops.
If one of these places accepts you, could you please write a review here on TAG?
I might visit Kofu again in the future as I have a friend that lives there, so it would be great to be able to visit a shop there that has accepted other foreigners.