Built the hood, used 2 6” diameter piping to create the 12” flue. It’s going to go straight out of the roof in my barn. My question is where did you guys get a 12” double walled pipe/flashing/storm collar/cap that fit the flue?

Try calling around to roofing suppliers in your area. Worst case scenario, make one!

It's been a very long time - I mostly work outside, and recently not at all, but fireplace and chimney supplies and box stores, maybe an industrial place. That large is a little harder to find - I seem to recall you can also take the seams apart on a smaller size of pipe and join them side to side but for double wall you're supposed to use an adapter so it doesn't put pressure on it and mess up the gap. Gets a little expensive at that diameter too, but cheaper than burning the barn down.

You might also consider having one that vents to the side then up to go through a wall rather than the roof if that's feasible and works better for you.