We need to replace a 12 ft length of metal vent in our basement; where

Please call in a licensed HVAC company to make the repair! This is a vital portion of both your heating & air conditioning system. A professional will not only make sure your ENTIRE system ( furnace & air conditioning ) works with the new components - but will remain on call for any problems you have with it - usually for up to one year. Get estimates from 3 HVAC licensed contractors. Your safe bet is to accept the mid-range bid. Asking for - & CALLING! - prior customers whom will provide you with a reference is also a very good idea. It really isn't a big deal guys - just pick up the phone/Google it....

You probably should be able to find it at the home improvement stores, if they don't carry it, they will be able to order it or tell you who carries it.

Big box diy. Hvac area by the sound of it...you will also need a special tape or short screws to connect the new. The special tape is metal. Makes nice silver stars for the Christmas tree.

Home Depot or Lowes maybe ???

Home Depot, Lowes, Menards. They come in 4' and 5' lengths. They will need to be snapped and screwed together.

There are many different sizes for vent,,,you could go to Ace Hardware or at Lowe's.. Be sure to bring and show them your metal vent...maybe it could be fix like welding.

Could you turn a screen door into a vent somehow?

i bet a welder or someone at a hardwood store could brainstorm something up for you. They have everything at their fingertips!