There are over 20,000 taxis in Istanbul. And they are bright yellow. Not exactly difficult to find.
In theory taxis here are relatively cheap. But like in many big cities you can come across drivers who want you to pay the dumb tourist fare. Not so cheap.
To avoid this, firstly make sure that the meter is always turned on. They try to hide it behind the gearstick, but don’t be afraid to seek it out at the beginning of the journey and at regular intervals. During the day the meter shows ‘gündüz’ and is 50% cheaper than ‘gece’ (midnight to 06.00).
Secondly, take a map of the city. Don’t let them take you the scenic route unless you’ve asked for it.
Lastly hand over and count out small notes one by one. Drivers often pretend you’ve given them 5 lira instead of 50 (they’re the same colour).