http://www.mcace.com/adapters.htm

http://www.mcace.com/shotguninserts.htm

I remember seeing these a long time ago, both for rifle and shotguns.

Does anyone know of any made for revolvers?

Yeah, I know they would free bore, so would the ones made for shotgun.

Thinking about how handy it would be to fire say .25 acp out of my 357 for say, dispatching animals, and not making as much noise out in the field. Who cares if it keyholes after 6 feet. Even my min loaded 38's still make a pretty loud pop.

I've used 000 (36cal) buckshot in a 38special with a 1gr of Bullseye for similar loads.  - cut and past from here - http://thefiringline.com/forums/showthread.php?t=390117

You'd be better off with a sub caliber, saboted bullet in the revolver case.  You won't have to have another caliber, another set of dies or a hard to find, and probably expensive cartridge to buy.  Plus they'd actually be accurate and get a decent velocity.