I really want to find an old, obscure novel or short story that I can adapt to the screen but I’m not enough of a book nerd to know about books that aren’t already movies.

Matthew “Movies” Thomason The Unpleasant Profession of Jonathan Hoag (Heinlein, 1942)

en.wikipedia.org/wiki/The_Unple…

Minimal special effects, small cast, potential for strong visuals, fun creepy story. Hasn’t been adapted AFAIK.