Installing a duct fan
The manufacturer's website lists several possibilities; from their FAQ for that range of fans:

Where do I get power from for the In-Line Duct Fan?
Here you have a number of options.
The easiest way to power your In-Line Duct Fan for automatic ON/OFF operation with both your furnace and air conditioner is to use the Suncourt DuctStat. Please view the DuctStat section of these FAQ's.
You may power the In-Line Duct Fan via a standard wall switch for manual ON/OFF operation. Running an In-Line Duct Fan continuously will not appreciably affect the life of the fan.
The next best is to connect to the wiring of your furnace. You must connect to the wire that provides power to your furnace blower. Consult the furnace wiring diagram, usually located on the inside of the panel covering the main furnace blower. Again make sure that all wiring conforms to all applicable standards and codes.
Also check that your furnace blower motor has a 110-120 Volt AC motor. Some larger furnaces have 220-240 Volt AC motors. Some of the newest high-end furnaces have DC motors. Do not connect the In-Line Duct Fan to either the 220-240 Volt AC or the DC type.