Located near Sermthai plaza. There're some bus lines to remoted area and also other provinces like Khon Kaen,Kalasin,Roi-Et and Surin.Bus to Bangkok separated to 3 classes; VIP class,First class and Second class.
There are cheaper ways to travel [although there are some new budget Thai airlines], but if your time is limited, take the plane!
There are regular flights from Bangkok that take about an hour BUT you still must get to Maha Sarakham. We went by a bus sent by our hotel. The road was potholed and the bus driver was a great fan of leaning on his horn.