Cincinnati here, just a mile or so from Kentucky. I'm 57 years old, and only in the past year or so have I heard the phrase "I'm not for sure." It is used where one would say "I'm not sure." The two are interchangeable. Mainly I've heard it used by African Americans in their 30's and 40's, but also lately from one European American guy who's 29 years old. I've questioned a few of my co-workers, all in their fifties, and they say they've heard it for years, mainly from socio-economically challenged African Americans. Maybe Mr. Tarentino got it right. Or maybe he invented it.