Range Hood and 7" Damper

We decided that the old range hood in our kitchen had to go. It was pretty dirty and had a "limp."

My wife wanted me to snap a photo of it as we pulled it down so we had it when we went to find a replacement. Of course, the replacements were designed differently now, and the fan was no longer mounted above the hood.

We did run into several problems. One was fitting the new unit in. When we found one small enough (that required only minimal carpentry), there were different fan sizes and we opted for the larger fan. Then for the 7" round duct(like we were replacing), there was no damper included. What's more, no one sold this damper that was a "needed accessory." Ultimately, we cleaned the old damper up using turpentine and installed it above the new range hood.

The final result was a new, vented range hood. When we looked upward when it was on, we could see the dampers on the new unit and the roof cap open. We installed the filter and felt good about a job well done.

So why doesn't Home Depot, Menards, Ace Hardware or the Dey appliance (distributing) store they referred us to have the 7" damper? They sell the range hood that says you need it. Then, nobody carries the stupid thing. The rectangular damper is even included (with the new range hood)! Does everyone use that one?