Master Flow 3-1/4 in. x 10 in. x 7 in. Hood Transition Fan Boot FB3.25X10X7 - The Home Depot
427 10-Inch Round Horizontal Transition for Range Hoods and Bath Ventilation Fans
Vent-A-Hood VP564 Galvanized Steel 18.5" x 11.25" x 12" Modern Themed Stainless Steel Cluster Blower Transition for Duct Size of 12" Round - VentingDirect.com
Deflect-O Galvanized Transition 3-1/4 " X 10 ", 6 ", 6 " Dia - Kitchen Tool Sets - Amazon.com
How to Install a Range Hood Vent Through The Ceiling
Broan Ducting and Installation Accessories: Transitions for Range Hoods | KitchenSource.com