I looked on Rockford but I don't see an adapter for this application? EDIT: I found this pic from the other site, its for an insert. I'd have to notch where the rod goes through, also it would be a round to round connection.
Any angled "Boot" works well if you need the angle. Available in stainless, steel or cast iron. Use self tapping screws or drill and tap to mount. If you can go straight up, simply notching a 8 inch connector pipe (crimped to reduce to correct size) works as well. To avoid interference with damper pry outward while crimping to prevent end from tapering inward becoming funnel shaped. Your picture doesn't show if the damper is handle or chain operated. If it is an early Insert with chain, the weight of damper plate opens damper so tightness on shaft and contact with plate inside becomes critical.
There isn't any "normally"... If installed in a tight fireplace the smoke will generally flow right up. If you remove the damper bar assembly then you can weld a collar ring on the stove, notch it first, reinstall the damper then go from there. "Some" Fishers had a ring on them although few and far between, most like yours did not and were direct slip in, no pipe required, That's why the surrounding metal came with it to stop any back flow in the house.
Correction; There was none originally. Use of larger chimney flues originally made for a fireplace allowed too much cooling of exhaust gasses resulting in excessive creosote formation. So they changed the code many years ago to require all fireplace inserts to be direct connected to a chimney liner extending all the way to the top. Installing without a direct connect to chimney liner is illegal. There is an exception if connected to a pipe with block off plate, but it is nearly impossible to connect this way.It also allows rising exhaust gasses to expand into original size flue causing creosote and poor performance. When installed as designed the front faceplate was sealed to the hearth face to prevent indoor air from leaking into the chimney cooling it even farther. ALL intake air must go through the Insert, not around it. Any smoke is pushed INTO the chimney flue by higher atmospheric pressure rushing into the flue where a low pressure area is created by rising exhaust gasses measured as draft. Standard can be found in NFPA 211 and International Code Council (ICC) code which adopts NFPA standard in the ICC Mechanical Code Chapter 8. (Most states have adopted this code statewide including Texas (2006 version) NFPA includes all of US. NFPA allows reduction of 1 inch which works better than the original 8 inch. Insulate the liner as well. If you're under ICC code that does not allow any reduction of connector pipe or flue smaller than appliance connector size. ICC also requires all appliances to be UL Listed. So a tag is required. This model actually works great reduced to 6, but is not legal to reduce that much. The reason for the larger outlet was to be able to connect to larger existing flues, which obviously didn't work out well. All Fisher Fireplace Series stoves had oversize outlets for this reason and work well reduced with an efficient chimney. An insulated straight up 6 inch liner drafts well even with an added baffle which should be added regardless of the size flue used. Adjust the baffle plate (smoke space) for square inch opening of flue used. Baffle is adjusted for chimney, not the stove.
It was required by NFPA since 1984 ! Man, I'm old. I thought it was just a few years ago........ Here is the Code requiring direct connection for you; ICC 801.10.3 here; CHAPTER 8 - CHIMNEYS AND VENTS | 2006 International Mechanical Code (5th Printing) | ICC publicACCESS
This insert has the handle damper, throat opening is 7-9/16". The install will be flex/insulated liner all the way up.