Are you specifically wanting a guided tour? The RS book offers many suggestions for private guides. SRM Travel is the company RS uses for his Turkey guides and they also do private tours. I did a food tour with them and it was fantastic. Do a quick forum search for other food tour recommendations. There have been some recent technical communication problems with Lale the owner but hopefully that will be resolved soon.
There are no tickets required for Hagia Sophia or the Blue Mosque. They are both working mosques and are open and closed for about 30-40 minutes for each prayer time. In May it will be approx. 3:24 a.m., 5:25 a.m., 1:10 p.m., 5:10 p.m. and 8:45 p.m., along with a longer Friday noon prayer time (so don't try to go midday on the 17th). Hagia Sophia has a security check which typically causes a long line to get in - it moves relatively fast but the lines are longest midday. I went to both Blue Mosque and Hagia Sophia on the morning just after the second morning prayer. There was hardly anyone at Blue Mosque and the Hagia Sophia was crowded but not as much as a return visit later in the day. Come prepared wearing appropriate clothing (no shorts, no bare shoulders), headscarves for the females, and a plastic bag to carry your shoes (easier than leaving in a cubbie).
Topkapi Palace does have an admission fee but I'm not sure if you can purchase in advance (I was on a RS tour and they provided museum passes). It is an enormous place - we did a partially guided tour and then had time on our own. The RS Istanbul guide offers a decent self-guided tour.