do you want true duals or just a y pipe on the end like the gen 3 duratecs or SHO's? if you want true duals it would have to come off of the headers. if you just want the look, then you can get the y-pipe and have an exhaust shop weld that on.
not sure on price, but you should be able to get one from an exhaust shop. Ebay is another good place to look. You can find one in the junkyard too, that woul be the cheapest, just make sure it's in good condition.