Many times I hear people ask "Is there a bus going from place X to place Z?" Well yes there probably is because there are busses going from almost any big town to any other big town 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?".
In North Thailand you will find the Green Bus company, that has 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). And all over Thailand you will find Nakhon Chai Air , that is is going from Si-Saket in North-East Thailand to Rayong along the Eastern Seaboard. That is a mighty long bus ride!
So yes do not worry, just go to the bus-station and ask.