I am a native speaker of American English from Southern California so my judgements are based on what I hear regularly. 1. I will probably not be there. - good 2. I probably will not be there. - good, but a bit awkward In spoken language 3. I won't probably be there. - not heard 4. I probably won't be there. - most common in spoken language 5. He probably does not know. - good, but in spoken speech it would more likely be heard "he probably doesn't know" 6. He does probably not know. - not heard 7. He does not probably know. - not heard 8. He doesn't probably know. - not heard

I don't know what is 'correct' - just what sounds right. To me, 2,4 and 5 are good. 1 is OK - in spoken language. The rest are not natural.