I have 8 " flue. Is there a pipe section that can go from 6"to 8"?

Yes, we have a Slip Increaser that goes from 6" to 8" in length. Model: 6-Inch Durablack, 3rd option down on the list of parts. 6- Inch Durablack If you have any further questions or need further assistance, please feel free to reach out to one of our experienced customer service representative and they would be happy to assist you. Thank you and have a wonderful day.