You need a state waterfowl stamp, which you can get just about anywhere hunting licenses or sold. Not sure if the online option is the current state stamp or the one that is valid from July 1, 2013 through June 30, 2014. If you already bought the 2012 stamp, it is valid to hunt the remainder of this waterfowl season. It actually expires on June 30, 2013. It costs $8.75. You also need a federal duck stamp. Only place I can consistently find those is at the post office. Game and Fish says you can also buy them at their offices. Same deal here. If you already bought the 2012 federal stamp, which is $15, it is also valid to hunt the remainder of this waterfowl season. It also expired on June 30, 2013.
The 2012-2013 state waterfowl regs are at http://www.azgfd.gov/regs/waterfowlregs.pdf
. Check pages 8-9, below the season dates. What stamp or stamps were you told you needed or didn't need?