The closest thing to a Flash Bang Grenade that can be used for Airsoft games can be ordered from  £1.65 according to its maker, Enola Gaye... "The Flash Grenade has been developed for Airsoft and Paintball players whos game play is in low light, urban, CQB etc.. It delivers a low bang and white flash that confuses your opponent allowing you time to enter the room and dispose of you newly confused foe.