Going with the bus in Thailand
While living here in Thailand I am still surprised that I see sometimes busses driving on the road with places names on it that I think "Really from that small town to Bangkok?".
The Green Bus company now has a new route going from Mae Sai in North Thailand to Bung Kan in North-East Thailand (via Chiang Rai, Uttradit, Phitsanulok and Khon Kaen).
Nakhon Chai Air is going from Si-Saket in North-East Thailand to Rayong along the Eastern Seaboard.
So yes do not worry, just go to the bus-station and ask. Or do a google search, although most bus company webs-sites in Thailand are in Thai only (or very useless English).