Well, if you ever find yourself stranded in the fair city of Prague and want to sit down, chill out, have a delicious beer and a joint, here is my suggestion.
From the airport, take the bus to the Metro station Dejvicka (20 mins). Hop on the subway to Jiri z Podeprad/JZP (15 mins). Walk approx 500 feet up the street Slavikova as indicated on the map and enter the bar on the right. The weed's pretty good and 220czk ($13) per gram. You can smoke inside and have a delicious Kozel beer. You can also head to the beer park in Riegrovy Sady (the green squareish thing) if you prefer smoking outside.