I expect that you won't find any reseller of Turkish SIM cards outside Turkey since they have very strict laws requiring all phones bought outside the country to be registered by their IMEI and if you use it with a local SIM card to be tied to it (see
.:CEIR-Central Equipment Identity Register). Registration costs 5 TL and sometimes they even ask for a invoice proving the phone was acquired legally. Furthermore you may only register one device per year, so if you carry a second phone or a USB modem you're already violating the law.
If you fail to register your phone it will be blacklisted and banned from all Turkish networks within few days up to three weeks at the latest. Once a phone is blacklisted it won't even work with a foreign SIM card.
Despite it doesn't circumvent the obligation to register your phone you might be interested in Turkcell Germany's prepaid SIM card sold in Germany aiming to Turks living in Germany, which does not only offer cheap calls from Germany to Turkey but also relatively low roaming rates for Turkey:
Turkcell Europe
Incoming calls (to your German number) are free up to 1000 mins/month (exceeding minutes cost € 0.09/min)
local calls and calls to Germany € 0.20/min
international calls to any other destination € 1.50/min
data costs € 0.42/MB + € 0.49 per day