Fitbit does not manufacture extension cords for its chargers. Fitbit chargers are made to be plugged into a USB port. Many stores sell USB to AC adapters, which would allow the Fitbit Flex to be plugged into a regular wall socket or a standard extension cord.
Chargers can be found on the Fitbit website for different Fitbit models, including the Fitbit Flex. All of these chargers are made specifically to plug into a USB port.
EBay lists at least one Fitbit charger for sale as a package with a USB-to-AC power adapter. On the same website, shoppers can find many different models of AC-to-USB adapters, and all of them should work with a standard USB plug and an AC wall socket or extension cord.
As seen on Amazon.com, standard AC extension cords come in many different lengths and can be used with any electronic device that plugs into an AC socket. In general, an extension cord from Amazon will cost less than $15. Charger cables for the Fitbit Flex retail for $20 on the Fitbit website, so purchasing an extension cable and an adapter on Amazon.com or eBay should be cheaper than purchasing a new charger cable directly from Fitbit.