Jeremiah 
I don't think there's a single go-to site. You pretty much have to track down things individually or in clusters organized with a common theme (like Mac products).
I found a website run buy a guy who sells vintage Mac parts. His price list might provide a jumping-off point for determining a price estimate. He might be overcharging, but at least you could get the price range.
http://myoldmac.net/apple-part-e.htm
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