8 Inch to 6 Inch HVAC Duct Reducer & Increaser - Galvanized Sheet Metal Ducting Connector for Airflow, Heating, Cooling, & Air Ventilation System Extra Strength and Fittings - Air Tool Fittings - Amazon.com
001-037 3" trans adapter for warm air
NORDFAB Reducer: 10 in x 4 in Duct Fitting Dia, 12 in Duct Fitting Lg - 40D560|8040025887 - Grainger
GE RAK6053 Duct Adapter for GE PTACs
Snap-in duct adapter for 4" ducting
Rheem Outlet Duct Adapter - GP Conservation
Two Way Duct Adapter
Duct Outlet Short Duct Reducer HVAC, 9" x 8" - Walmart.com