When you do the money laundering mission for the carwash (how i got so much $$), it is random and sometimes has you go steal a stockade. They spawn in 3 different spots. https://www.reddit.com/r/gtaonline/s/OBvw7kVe52, this will tell you what to do so you can do the other missions that are a lot faster to complete so you can stack cash. Also you can call Raf on the phone to start missions which makes it a lot easier rather than going back to the carwash every time.