Amazing city... just recently spent a week there...
I didn't take my board with me, so I don't really know too much about skating there... really crowded in most places etc.
My suggestion would be to go to Kadik?y on the Asian side of the city... saw 2 skateshops there, I'm sure they'd be able to help you out regarding spots/park or whatever... I remember that one shop was called Freedom, looked pretty legit.
Anyways, enjoy your time there! Istanbul has a lot to offer, that's for sure...