Every year my uncle will hold a banquet for all of our relatives at Rakuzen the japanese restaurant. He will order a whole frozen salmon fish from the chef. The chef will fillet the salmon for sashimi while the skin used for tempura, and the rest goes into miso sukiyaki. Not sure if this kinda salmon you are looking for. Otherwise many groceries sell frozen salmon as well. Farm raised salmon are fed with pellets for its orange color but i guess that's still consider natural since all wild salmon also get their color from crustaceans.