Ah the quest for taze kişniş. I know it well.
The Tuesday market guy who occasionally sells it is at the river end of the fruit and veg tunnel. So with your back to the river and the footbridge also just behind you look down the right side of the tunnel. He is near you on the left (of the right hand tunnel) just before the tunnel ends and becomes sky. Largish bloke with big tummy, round face in his late 30s perhaps. You have to ask him for it (tarzay kishnish cos it isn't always on top of his stall.
Failing that, the big Migros in the centrum have had it in film bags. Occasionally.
Failing that Akgünler Wholesale Manav on the Taşyaka road will order it in for you from Antalya.