Flahne's Under Desk scene (requires having talked about her race) and both Exhibition scenes (requires having done the Under Desk scene) all have Exhibitionism gains without requiring any amount of Exhibitionism to access, so she's a good early game source. Erra's Public Pets are easier to farm, but require having had her Past, Relationships and Dee talks, which in turn require having sex with her at least thrice.
If you have a vagina and aren't pregnant, Miscreant Manor on New Texas gives a significant boost to Exhibitionism, though if you want to farm Exhibitionism that way I'd recommend taking some Sterilex beforehand since there's a very good chance of getting pregnant each time it's used.
The hardlight jockstrap is transparent, which means wearing it with clothes/armour that expose your ass/crotch or are also transparent, or not wearing any clothes/armour at all, will cause your ass and/or genitals to be visible, which in turn will cause your Exhibitionism to steadily rise each day up to a certain point (depending on how much of your body is visible). Though if you want to raise Exhibitionism that way, it'll be quickest to go fully nude.