Overnight land-based travel from Istanbul (Turkey) to Skopje (Macedonia)?

I am looking to travel between Istanbul and Skopje overland during August (2015) and wondered what the best way to do this would be?

My initial plan was to take an overnight bus as I've found evidence of an overnight bus http://www.balkanviator.com/en/bus-timetables/istanbul-tur/skopje-mkd/15.08.2015. I am interested in both 21:00 departures on the 15th.

But when you click on "buy" it tells you that that part of the website hasn't been built. And then going to the actual bus company's website at the Istanbul end (http://www.alparturizm.com.tr/tr/anasayfa) it seemingly times for one direction, Macedonia -> Istanbul, and not the other way around. Also in the times there's no mention of Istanbul or Skopje. For the other 21:00 departure the bus company which runs that don't seem to have a website (Googled 'Jadran Istanbul').

I wondered if it was a sensible idea to take an overnight train to Sofia (Bulgaria) and getting a bus from there? Though the man in seat 61 informs me that there's a bus replacement service to Istanbul. So this looks less attractive though possibly still doable.

Side note: it's mad that a city of 14 million people have had no rail connection to the rest of Europe since 2012 (the time of writing this summer 2015) and have permanently closed a mainline station in the centre of Istanbul!

Buying a ticket in advance is not crucial, though I would like to know:

1. if the timetable I've found is reliable,
2. the likely availability of tickets (is there any chance they'll sell out?), and,
3. and crucially, where can I buy tickets in Istanbul?