Looking for a reputable source to purchase tickets in advance for Topkapi and Hagia Sophia. Any recommendations? (We will have a guide, but admission is not included)
This is the official source for many of the Museums in Turkey:
https://muze.gen.tr/Museums
Topkapi Palace: https://muze.gen.tr/muze-detay/topkapi
Hagia Sophia: https://muze.gen.tr/muze-detay/ayasofya
Links to buy tickets are on the page for each of the museums.
ETA: Interestingly, both the Hagia Sophia and the Topkapi Palace pages direct you to GetYourGuide to choose from tickets and ticket with tour options.
The official site also includes various Museum Passes for Turkey and for Istanbul, some with transit.
https://muze.gen.tr/MuseumPasses
They mention free entry to the Blue Mosque, which is currently free to everyone, without a ticket or pass.
The skip the line tickets are new-ish feature since they changed the format for entering Hagia Sophia. For the online tickets you pay a small premium ($3 or so) for the convenience. Will you need to get admission ticket for the guide too? Double check so you aren't left without them when you enter.