You get it from Belle after doing all the events in the bath house.
As HotSausageGirl said: Bella will gift it to you after doing ALL the events in the bathhouse:
Women's Baths;
1. Wash up > Make a show.
Mixed Baths;
2. Ask Annie to try a bukkake.
3. Kneel near the line of men waiting for BJs.
4. Go into the water...
Men's Baths;
5. Enters the men's baths...
Return to Bella once you have done all the events and she will have a new dialogue option "Bella has a surprise for you". She will then give you the micro bikini.