All through Buriram, Surin and Sissaket you'll find Khmer speaking villages.
Southern areas of those provinces will have more Khmer villages but also Issan Laos, Thai Korat and Gui.
Sadly, these languages, possibly with the exception of Issan Laos, are dying languages. My wife speaks Khmer to her family but our daughter, although fully understanding Khmer, chooses to speak Thai.