Most of the neighbourhoods in Istanbul has at least a street for antique, vintage shops. Called “Bit Pazarı” in Turkish.

Recommended 3 places:
-Çukurcuma in Beyoğlu. 5 Minute walk from Galatasaray Highschool on Istiklal Street.

-Kadikoy Antique shops street. Central Kadikoy

-Without doubt, you should be awake, not to miss any hidden antique shops around Grand Bazaar and Spice Bazaar.

Thanks to @Ellie_Tennant for asking this question on twitter.