Why don't you just open a bank account with someone like Nationwide, which will only charge you 1% for cash transactions outside Europe? - then you don't have to take cash at all, you can just go to the ATM at the airport the other end and take out what you need to get started. That's what we did in Morocca in the spring, we never take foreign currency with us, then we can't risk losing it. If you should be staying in the part of Turkey which is in the EU then Nationwide don't charge you at all (although bear in mind the operators of the ATM might levy a withdrawal fee).