Does anyone know where I can find detailed information on the Internet about buses between Ubon and say Sisaket or Rattanaburi? Plenty of information via search about Bangkok-based buses to each of those places but I cannot find anything more local for travel between those places

There is nothing on the internet.

Ubon to Sisaket:  van service only frequent departures throughout the day from about 6 a.m. to about 6 p.m. Running time is just under an hour.

Sisaket to Rattanaburi: 6 vans per day, every 2  hr. from 8 a.m to 6 p.m. Running time is about 1 to 1.5 hrs.

Ubon to Rattanaburi (by bus) - this is not the most direct route from Ubon to Bangkok,so service is infrequent. I believe there is a bus leaving Ubon at about 6 p.m. daily.