If you're looking for a loosie in Center City, Philadelphia, just go to 15th & Market Streets, usually near one of the entrances to the EL -- either the one beneath the Clothespin or the other on the north side of Market Street.
To be clear, selling loosies is against the law in Philadelphia. If a loosie is sold to anyone of any age, it can result in a $250 penalty.
None of that seems to stop the guys hanging around 15th & Market Streets, though, likely due to 1) the demand -- Philadelphia has the highest adult smoking rate among the 10 largest U.S. cities, at 25.2% -- and 2) limited enforcement capabilities.
I see and hear those guys almost every time I walk to and from Philadelphia Family Court: "loosie, loosie!"