They are advertised all the time, alot of people just don't pay attention.
It will say, something to the effect, fully furnished, daily rate, weekly rate, come and go as you please. Providers will rent these for a day or two and then get another one somewhere else in the city. Here in DC in Chinatown there are tons of them available. They will take girls to the airport also.
I'm not sure how safe they are really. I'd be careful.