Replacement for salmon in Istanbul?
Fresh salmon steaks and fillets, usually imported from Norway, are generally available at the larger supermarkets...try Migros (especially a big one) or, if they've arrived in Istanbul yet, Kipa (operated by the UKs Tesco). Make sure you get the stuff from Norway, otherwise you might be buying salmon-trout, a fresh-water fish (which is usually sold whole).
Beware: for some reason, fish doesn't seem to come scaled ... so be prepared to scale the salmon yourself. Also, IMO the Norwegian salmon I've tried here doesn't have the flavour of Canadian salmon (and, I assume, what you're used to in the US).