6 to 4 Inch Duct Reducer, Hon&Guan Duct Adapter 6 to 4 PVC Reducer, Duct Increaser/Reducer for Grow Tent HVAC Ventilation System. (150 to 100mm) - Amazon.com
Vent Systems 8" to 6" Inch Duct Reducer Adapter Connector Fitting Pipe Increaser Reducer Cone for HVAC Ventilation Systems, ABS Plastic PVC Pipe 200 to 150 mm - Amazon.com
NDS PVC S&D Increaser/Reducer Coupling, 4 in. Hub X 6 in. Hub 6P07 - The Home Depot
Charlotte Pipe 6 in. x 8 in. DWV PVC Pipe Increaser Reducer Bush PVC 00102 1900 - The Home Depot
HeatFab 8'' to 6'' No Crimp Reducer - 2836B
Fiberglass-PVC Swimming Pool Concentric Reducer - 6 in outlet-8 in inlet