There seems to not be much information online about catching a bus from Burgas to Istanbul so I thought I would update the situation! Hopefully makes it a bit easier for people out there!
Most information online is difficult to come by but there is a central business station on Burgas which is new, has free WiFi and air-con and all the bus operators now have offices here. You can look at some schedules in advance and book tickets here but not all companies maintain an online presence and even in peak season if you turn up before midday (most buses leave between 1-3 pm) you will be able to find a free seat.
Ticket prices are around 45 LEV. We decided to go with Nisikli as you could book online. Old information online says the bus stops is 10 mins away from the station but this is no longer true. All buses leave from this central terminal and Nisikli maintain an office here also. If you are coming in the same day you can read my guide from catching the bus from Nesebar to Burgas here but it’s also worth noting Nisikli route actually takes it directly through Sunny beach so you could catch directly from there and save yourself some hassle!
Nisikli were unresponsive to emails and calls before hand, and the listed number and phone on there website do not work. Neither does their online chat function and it looks like they have been building their website for many years now but you can book online … just be sure to save your ticket as weirdly they will not email to you. However the bus came on time and was modern, clean, great air-conditioning and has free ice cream and coffee!
The border creasing was simple and painless and you only have to get off the bus once. Entire trip should take 6-7 hours but delays can easily occur and we were 1.5 hours late to arrive in Istanbul which is important if you have onwards plans.
The destination in Istanbul will generally be the International Bus Station Istanbul (Metro stop: Otogar) but confirm with the company. ATMS and food are very accessible here with plenty of options, though it will be a very intense introduction to this beautiful city.