Country buses in France are run by "departement" (county) councils. You need to find out which departement your destination is in, and then search online for the name of the departement and either "transports" or "autobus".
Usually this will bring up the relevant website quickly but they will be in French with very rarely any English. You can often find information even if your French is limited by looking for the words "horaire" (timetable), "plan" (map), "reseau" (route network) or "tarification" (fares).
Be warned that there is very little in the way of longer-distance bus services across departement boundaries. Also bus schedules can be aimed strictly at taking schoolchildren and elderly shoppers from villages into the market towns in the morning and back again in the evening.