I just went to Menards and bought double walled wood stove exhaust duct and it works GREAT! Still not cheap but a couple hundred dollars wasn't too bad. I can't remember exactly how much it cost though, I installed it over 4 years ago.
I'm in downtown Hartford, 22 Pratt Street.
As far as your exhaust pipe is concern, at the very least, you need to meet your local fire code. But above that, you also need to make sure if you have a roaster fire and the fire works it's way up your pipe, the fire is not going to melt your pipe and burn the building down. You should over engineered your system for the worst case scenario. If you are not sure, you can give Diedrich a call and see what they suggest. Naomi at Diedrich's tech support is very helpful.