5.11 "covert" dress shirt for $14.99 at LAPolicegear; less than half of what I've seen them for elsewhere:
http://www.lapolicegear.com/511-covert-dress-shirt.html
Never tried them myself (largely because I'm cheap), but at this price I ordered a couple out of curiosity. Apparently some of the buttons are normal/real, some of them are sewn on for appearance only, with breakaway snaps beneath them for accessing either a holster or the the shirt's internal pockets.