I hear Vietnamese relatives of mine say "có thể" and "không có thể" very often, and they are all native speakers. Many phrases are probably quite different coming from different areas of Vietnam. I agree that "Không có thể" should also be accepted.
Haha my reference would be my assimil book, lesson 58, footnote 7. It basically says that không thể can also be encountered as không có thể. However, they are using không thể in the examples so... I suppose it's seldom used ^_^