if you go cambodia and back on the same day, the cam immigration might deny your exit as they might require you to stay 1 day. tourist visa can only be from thai embassy in other country, not in thai. you might be able to get 30 days extension at your local immigration with passport, a passport photo, tm30.
Dont think there is any agent there now, poi pet is the worst place to do visa ran. Even agents avoid it now. (the cam side might not stamp you out if you do not stay for 1 day in cam.)