Port Authority Taxi Stand
There is one main stand for Port Authority on 8th Avenue between 41st and 42nd Street. You can also try to hail a cab on the street though depending on time of day, that can be more challenging. There are many ride share services as well that you can utilize such as Uber, Lyft, and a few others.
Outside the main concourse on 8th Avenue between 41st and 42nd Street.
See the yellow stand with lights in the Google Map Street View.