Thank you so much Kaan! Just got back from the market in Arnavutkoy - would have NEVER found it without your help!
Here is the list:
Arnavutkoy: Tuesdays on Ogul Sok. I got it from Joy's website.
4. Levent: Tuesdays - couldn't find the exact address, but it's somewhere around the bus stops for buses go towards Sariyer destination.
Rumeli Hisar: Saturdays (don't know the exact address, but I'm sure it's right by the hisar)