They're around if you really want one, ammo too:
http://www.shotgunworld.com/bbs/viewtopic.php?t=86820
However, given the easy availability of 28 gauge and a good choice of ammo, I'd think you can do anything with a 28 that you can do with a 24 or 32.
In fact, if I had it all to do over again, I think I'd own only 28 ga guns. light and sweet for quail, plenty for clay targets, and 1 oz. loads which will kill pheasants and ducks.
The 24 is "almost" a 28, and the 32 is "almost" a .410.
But, if you want ti shoot something *different*, you can still find 14 ga and ammo for them. Not that they're any dofferent from a 16.