Of course, the oldest standby for getting drugs without money, besides maybe robbing someone who has the drugs you want, is to rent out your ass or pussy for sex. Neither of these first two methods are good for your health if you get hurt in a botched robbery or pick up an STD, respectively.
Thus, I would recommend finding a good dealer who doesn't have a car and either driving him around on errands such as re-upping or outright letting him borrow your car while you chill at his crib or somewhere, provided that you have good car insurance and are not afraid of notifying authorities should your wheels end up in a chop shop.