the pipe has to be heatn glo
it is not smoke pipe or b-vent it is intake outer with exhaust iner pipe
and has to be what is designed for the unit
contact heatnglo for replacement and get the sealer for it also
Good luck finding some old 8-5/8x5 pipe. No, you cannot use alternative pipe such as their DVP series pipe which is 8x5".
You can contact HG Tech Services as they might know of a newly revealed stash somewhere but otherwise, you are out of luck.
You ought to think about discarding that unit. It is about 15 yrs old, which is long for fireplaces.
FYI, the 8 5/8 OD was from Ron Shimek's original patent on the direct vent fireplace back in '87. He made that odd size so he could control the pipe as it is tested as a system. Current HHT product uses their own DVP pipe rolled in the Heatilator factory in Mt. Pleasant, Ia. Been there many times.
Keep the fire inside the fireplace.