You've circled one end of the extractor. The other end is protruding from the breech end of the barrels. There should be a small screw that fits into a notch on the extractor rod which allows the extractor to move in and out but prevents it from coming out.

Try Numrich (www.e-gunparts.com), e-Bay and even googling Flues extractor.

P.S. I'm assuming it is an extractor. If the breech end is split it is an ejector (which is more complicated and probably more difficult to find).

Good luck,
Phil