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I know you can do it with pellet, never looked into doing it with wood.
Anyway, for the question, any dealer who sells direct vent gas inserst should be able to get you some 3" aluminum flex liner. Thats what most of them use for venting.
I'd be wary of thin aluminum in the same flue, especially if the primary liner isn't insulated. However, they make stainless 3 and 4" flex for pellet stoves.
You do not want to use it if they are in the same flu. Stainless steel liners are rated to 2100 degrees whereas your aluminum is only rated to approx 550.