I am a sales rep and daily travel the state. I know most of it pretty well. There are abandoned vehicles everywhere, there is no challenge to find them. Every farm will have them. I do know where there is a cache of 1920s vehicles sitting in a grove but am reluctant to post its position. As for towns, they too are everywhere. Some that quickly come to mind are Hetland, Butler, Cottonwood, St. Onge, White Rock. Manchester is a somewhat interesting spot because there used to be a town there, now there are only steps and driveways. Tornado wiped out entire town. Same with Spencer, although they partially rebuilt. Most landowners will allow access, but I do make a point to at least find the owner to ask, especially if there are cattle out. Finding the owner is not always easy as there are many places there aren't any occupied houses for several miles.
Kent in SD