Check out gunbroker.com. I had the opportunity a couple of years ago to buy one at a gun show for $900.00,guy wouldnt budge off the price so I didnt get it.Wish I would have now. The build quality is good IMO.You wont be dissappointed.
The SPAS-12 is my favorite gun of all time, I plan on buying one as soon as I have the cash to. But here is a good site about them it contains detailed information about the different types of guns and it has classifieds on it. And good luck with your search!