IMPERIAL 6-in dia Galvanized Steel Rain Cap in the Duct Caps department at Lowes.com
Roof Rain Cap(18 Inch) (GCT 18) - HVAC Express
8 Inch Round Roof Rain Cap HVAC Vent Galvanized Steel All Weather Chimney Cap Roof Top Round Roof Vent with Rubber Gasket for Perfect Insulation Vent Cover (8'' Inch) - Amazon.com
Oatey 3 in. - 4 in. Mushroom Vent Pipe Cap 43805 - The Home Depot