I hear both (and their negatives: "I'm not sure" and "I'm not for sure"). I want to classify the "for sure" variety as regional Southern, since that's the context I most often hear it.
For example, take the "gimp" scene from Pulp Fiction. (Not that usages in the movies prove anything, mind you, but this is an example most people will recognize.) The character Zed is asked which of the captives he wants to "do" first, and he replies, with a distinct Southern accent, "I ain't for sure yet."
Can anyone point to evidence that this usage is regional, or class-based, or particular to any group of speakers?
EDIT:
Since @MrHen asked for more examples, I googled "I'm not for sure" and picked these off the first page:
"I'm really not for sure ~ what to do with you."
"I'm not for sure if I'm allowed to bump this …"
I think it's heard more in the negative than the positive. But I do hear it a lot. In the positive form, it's usually used in the form of a question? "Are you for sure?" instead of "Are you sure?"
UPDATE
I was listening to NPR last night and someone from Alabama who was being interviewed used "for sure" in the positive, as in "I was for sure that I wanted to do that."
Best Answer
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.