Based on your location it seems like you might be looking for the Canadian site.
Would you like to continue, or switch to the Canadian site? - US
Ducts, Dampers, Adapters & Caps
Broan-NuTone® Roof Cap, Steel, Black, 3-1/4" x 10" or up to 8" round duct
Select the Broan-NuTone® 634 Roof Cap to provide reliable rooftop ventilation. Features a black enamel finish and is designed for use with 3-1/4" x 10" or up to 8" round duct. Includes a built-in back-draft damper, and protective bird screen to prevent small pests from entering duct.