3 HOURS ON QNS. STREET ALL IT TAKES TO GET FORGED PAPERS IN SECURITY SHOCKER CAUGHT IN ACT: Post reporter Douglas Montero (left) shakes hands with shady documents dealer "Flaco" Photo: Matthew McDermott NEW YORK POST INVESTIGATION For an illegal immigrant desperately seeking to work in NY City, getting a high-quality forged green card or Social Security card is almost as easy as buying a soft-shell taco....... this reporter arranged a face-to-face meeting in Queens with a 20-something Hispanic man known in the neighborhood as "Flaco" - the Spanish word for thin......Flaco didn't appear threatening at all - surprising, considering...