How can you find parts for a discontinued bike - in this case, a 2004 Specialized mountain bike I'm looking for the rear shock link for a 2004 Specialized FSR XC. Specialized doesn't have them; and so far I haven't been able to find that link anywhere! 

Can anyone guide me in the right direction in finding parts for discontinued bicycles? Where should I be looking for parts?
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.