Is it possible to obtain advance tickets to sights such as Blue Mosque, Hagia Sofiya, Topkapi Palace and Harem, and Cisterns without booking a tour guide?
Have you bought the Istanbul Rick Steves book. If not, please do so as it is a wealth of information and will help you answer your questions. I took the week long Istanbul tour. We went on our own to the Cisterns and just walked in and bought our ticket. Not a problem. As for the Blue Mosque, there is no ticket, it is free and we walked in to see it. As for the Hagia Sofiya there is a web site you can go on to see how much it is and any information you need. You don't need to book a tour guide to see the places you listed. But a good guide book is very helpful. The Topkapi Palace also has web site for tickets and times. Istanbul is an amazing city and with a little research and a good guide book it is not too hard to get around and see all the sights. Have a wonderful time. Best, Ann
This is an addendum to Ann's comment: There is no fee charged to see the Blue Mosque. However, there is a kiosk just outside of the exit where a person requests donations for the upkeep/maintenance of the mosque. This was a very special place to visit so I gave him a small donation and received a receipt (not sure if I can use this for tax purposes in the USA). However, it seemed like no one else made a donation which is sad given the tourist value of the site if nothing else.