Last time I was in Istanbul (ok, the only time ) there were a group of photo shops in a multi-storey building, just near the Sirkeci tramstop in Ankara Cd, at the Southern end of the Galata bridge. At least two had darkroom supplies. Failing that, shipping from Germany shouldn't be too bad - try Macodirect, they seem to have Tetenal and Foma powdered fixers.
Alternate sources of sodium-hyposulphate might include local suppliers of swimming-pool chemicals too, it would be cheap but the smallest quantity might be 10kg !
Edit: You might also be able to get help with possible suppliers from this darkroom www.galatafotografhanesi.com.