I find them by finding them. Local tourist information centers may know about them, I tend to stop at those.
Don't expect places like that to be in directories, even the online lists have standards about facility size and what needs to be available. Other sorts of places that won't show up will be private (church, social group, tribal) campgrounds, and often the RV parks that cater to seasonal workers, particularly those doing long-term stays (unless you find a list particular to that market), and most lists tend to also exclude trailer parks that have a few RV sites.
My town has one RV park and a nearby state park that makes most of the lists or directories. Some include one or two of the COE facilities at a local lake, but there are additional campgrounds that do not get listed because they are too primitive. There are at least two RV parks that never get listed because they are also trailer parks.
