A web search for "oval 8 inch chimney liner" comes up with most showing 4 3/4" on the narrow side. There are many retailers and manufacturers to choose from.
You "should" use an 8 inch liner, yes. But a 6 inch flue will also work, depending on other factors and use.
1) Do you plan on open door burning with a screen?
2) Is the installation subject to inspection?
3) Insurance company or local codes involved?
4) Is this a "full size Insert" measuring 21" across front that projects from hearth, and approx 32 1/2 deep with out ash fender (shelf) ?
I ask these due to technically being against code to reduce flue or pipe size from the stove outlet size...... However, these codes were written to avoid reducing the already correct size flue or pipe to avoid a restriction of exhaust flow.
The Insert was made with an oversize outlet like all the "Fireplace Series" double door models to prevent smoke roll in during open door burning with a screen in place. The single door models all use 6 inch due to the inability of open door burning. The success of the "air tight" stove was due to the efficiency of the large firebox with smaller vent. (this later proved to be the problem not passing stricter smoke particulate laws. A larger outlet could have been added taking away from efficiency and burning clean, but the company refused to make an inefficient stove) The largest of the single door series, the Papa Bear, has a slightly larger cubic inch firebox area than the Insert, and was designed for 6 inch pipe and flue. (they were also available with side or rear outlet and obviously draw well with the outlet in those more favorable positions as well) So the cubic inch area of each firebox is compatible with the 6 inch outlet and flue,
when used as a radiant heater and NOT in Fireplace mode with open doors. (the manual recommends open door burning with screen attached for 5 to 10 minutes daily since the original installation used the much larger flue that the insert was made to vent into and run much cooler than a lined chimney)
That said, it will physically work, and most have no smoke roll in problems when opening doors with the reduced flue size. You will not have the capability of quite as fast heating up due to the exhaust restriction, but once up to temp they work quite well IF codes that are written from NFPA 211 not allowing the reduction in size is not a factor. There are MANY double door 8 inch outlet stoves in use reduced down to 6 and even continue to work well with a smoke shelf baffle installed as they were equipped in the later versions.
When adding a smoke shelf baffle, (the Insert never had) normally you must make sure the square inch area of smoke space above the baffle is at least as much as the outlet square inch area of the stove. (50.25 square inches with 8 inch outlet) It has been found when reducing to a 6 inch flue, the smoke space can be reduced to the smaller (28.25) square inch area, equal to a 43% reduction in smoke space area with no smoke roll in problems. (with an insulated flue)
There are a few threads and members on the Forum with that set up. Here's a current thread on the subject;
https://www.hearth.com/talk/threads/grandma-fisher.120270/
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