Ventilation - Insulated Flex Duct 6in
Assuming the vent pipes slope down to the HRV then insulated is not required because any condensation water will come out of the HRV drain. Also uninsulated pipe takes less space in the wall/wainscoting.
http://www.supplyhouse.com/Hart-Cooley-44785-6-x-25-F090-Silver-Air-Connector $23.86 25 feet of 6" duct.
Even better than using flexible pipe is to use 6" PVC sewer pipe available from Lowes.