Istanbul's city centre is highly tourist-oriented, so I'd probably check book shops in Sultanahmet or on Istiklal street. I recall that there's a book shop right next to Sultanahmet tram station, in the same building as McDonalds; I definitely bought city maps in English there, not sure about Lonely Planet.
D&R Bookstores are located throughout Istanbul, and they generally stock Lonely Planet books in English - at least for the Istanbul and Turkey books.
They also have stores at the airport (airside) that definitely stock the English versions, however if you are already in Istanbul then this likely won't help you.
Unfortunately their website is in Turkish ...