The above picture is an example of an 8″ rigid chimney liner with flexible chimney liner attached. The liner is upside down in the picture, but you can see then the flexible chimney liner is oval so it can fit thru the fireplace damper area with no problems. If you want the best liner for your fireplace insert yet have to make that bend around the smoke shelf, use rigid chimney liner for the straight section of the chimney and smooth inner wall flexible chimney liner for the bottom few feet.
Another option is using a heavy gauge flexible liner such as the Flex King HD. It is not as flexible as the other Flex King products but it uses a much thicker stainless steel. This is the best flexible liner for wood stoves.
Make sure you do everything you can to make sure you and your family is safe when burning a fire inside the home. Please read this article to help you make wise decisions in purchasing a chimney liner.