You would probably be better off scratch-building that. A couple of pipes for the barrels, and the handle off of a toy western rifle. The rest is pretty self evident. Good luck though!
Volpin props made a stellar Force-a-Nature, and he does a pretty good job of detailing how he did it, plenty of pictures. I know it isn't the Scattergun you are looking for, but the two are very similar, and I'm sure that similar construction practices could be used: