I'm similarly stuck, soft locked as Stacy because I don't have money to get into the bath house, and can't find any way to get more.
I spent the 100 coins you find on the healing items at the diner (because I was expecting to get attacked by the bully when I left), found the 5 coins in the machine there, and used that to get into the onsen, so then when switching to Stacy there's no money to be found in the remaining locations, and the merchant at the diner doesn't buy anything.