As do I - I just wish I could stumble upon someone doing it
The best way I've found is to go out with a short(ish) skirt/dress on and no panties (as I do almost every day) and then whenever I get a chance, be it in lectures, on a bus, in a changing room... I can just play with my pussy. It's wonderful cumming in places like that, but I also really like just fondline myself throughout the day and then cummnig really hard (or many times) later on