Turkey’s bus network is comprehensive, affordable, safe and comfortable.
From (dated) experience, I remember, however, that the further east you go, the harder it is to get reliable information on connections and time tables. Also, I can imagine that, with the current less-than-ideal situation in Turkey’s southeast, scheduled busses, here, are less frequent than before.
Seat61’s information on how to get to Tabriz from Tatvan seems reasonable. Alternatively, you could also travel through Dogubeyazit, for example from Erzerum or Trabzon. However, when I made the connection at Dogubeyazit, there was no cross-border public transport. I had to get a local dolmus to the border and another one on the other side.
I tried Rome2Rio, but they give a public transport connection that travels through Azerbaijan’s Nakchivan province. Though Azerbaijan has made it much easier to get a visa, I am not yet convinced these new tourist visas include Nakchivan (which is a stretch of land on the other side of Armenia).
Credit:stackoverflow.com‘