How can you find parts for a discontinued bike - in this case, a 2004 Specialized mountain bike
First, try looking for a "scrap" FSR to salvage the linkage from.

Second, BETD have produced aftermarket suspension components for various Specialized, GT, Kona etc designs. They might not have the item you need in stock, but they should still have the files and machinery necessary to make one for you, at a price. http://www.mountainbikecomponents.co.uk/specialized-fsr-04-06-linkage

If you can find some engineering workshop near you, they might be able to copy your damaged linkage. A basic non-adjustable linkage should be achievable using any 3-axis mill.