After your second time making Shiba wear the cat costume during foreplay, Haato should comment that the costume looks nice but a dog costume would be much more fitting. After that just keep interacting with Shiba during the day as usual and one of the day events should have the pink bear from the hotel telling Haato that he's got the goods he wants. It will happen naturally so don't panic! (I was panicking about how to get it too lol). After this event, go to the hotel at night and talk to the bear. He'll let you buy the dog costume for a price of 20.000. The costume isn't a foreplay option, keep interacting with Shiba during the day and if you've already bought the dog costume an event will trigger with Haato making Shiba wear the costume :)