Until fairly recently, folk art was mainly associated with functional/utilitarian items. Folk artists didn't paint something on a scrap of roofing tin hoping someone would buy it to hang on a wall. They painted something on a wooden napkin holder hoping someone who needed a napkin holder might pay a bit extra if it had a painting on it.

So, one way to approach searching out information about folk art would be to ignore the "art" and search for examples of items which folk artists would have used as media.