Frank,
Before answering your question, I wanted you to think once more of your desire to live in Istanbul! Just a suggestion..

About your question, unfortunately, law profession and practising as an attorney by registering to a Bar requires Turkish Citizenship(or being a Turkish subject).Perhaps, you may find a chance to work as a consultant for a law firm, yet you'd be deprived of the right to represent your clients before the Courts.

Goodluck