They have them at my Lowes and HD. If you go to the kitchen dept where the range hood are, many times that material is right there. Ask an associate.
You may need to transition the 3.25x10 to a round duct to find a good wall vent, but those transition pieces are everywhere. Maybe instead of going right into the wall from the OTR, you can go up into the cabinet above where you can transition to a round duct and send that right out of the wall with the vent. - http://www.lowes.com/lowes/lkn?actio...377&lpage=none