Actually mate I have a Jimmy Hart figure somewhere packed away loose, so long as the kids didn't bury it or melt it down I should have the megaphone in question. However the one I have i am pretty sure isn't plain white however if you are doing custom work you could always paint it white.
Edit: The one that was shown in your pic is from the Jimmy Hart solo figure - the one I have was from the Hart Foundation threepack, due to his outfit being a pattern of hearts the megaphone is the game however I have included a pic so you can decide - I believe it is exact same as the one in your pic just a repaint.
Not sure what size you require but that is as close as I think you will get. Like noted above there was a police academy one however from memory it was much larger than this one as you could actually blow in it like a kazoo.
