the one I know for sure about involved visitation.

wives and girlfriends are asked to smuggle something in for a significant other inside of themselves. a toilet trip later it gets removed and passed off, and voila. even a small amount can curry quite a bit of favor and of course cash. because of the risk involved, the price is well marked up.

seems like the guards would have to be on the take, though, because I'm not sure how the inmate is supposed to get it from the visitation room back to his cell.

I'm sure they have something figured out, anyone who can light a cigarette with an outlet is bound to be a crafty cat.