For me NakornChai Air is the ONLY BUS I use in Thailand. IMHO the best service.
Their Phone Number is 1624 (or : (+66) 0-2939-4999) and get a Thai speaker to do the talking. I am not sure if they have an English option in their Call Centre, as I have never tried it. Edit: Tried it and they have, just press 9)
AFAIK, there are at least 5 (NCA) buses daily from Bangkok to Buriram.
Edit: As Starmans link points out there 11 buses daily and NOT 5
The leave from NCA their own terminal which is located here: (
https://goo.gl/maps/KUZ3bQ66QdN2)
and they then travel towards Morchit and then on-wards.
If NCA doesn't work out, just go to Morchit Bus Station and get a bus from Morchit that stops (everywhere) and will eventually get you into Buriram. More then enough options available.