hvac ductwork question

I ordered some wood return air duct vents to replace my old metal ones. I was concerned that the new vents did not move the same amount of air so I ordered a larger vent than what is in the wall right now. When I saw the new wood vents I was correct in worrying but the larger size makes things right. My problem is that there is a metal band that runs around the 1/2 inch edge of the drywall of the old hole. I don't know why it is there. Where can I go so I can get some new metal bands in the proper size?
Thanks,
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Jason wrote:

Hi, Probably to protect the edge of drywall and prevent dust from getting sucked into duct? I saw such thing made with plastic like molding. But there could be metal one as well.
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