I cut and soldered my own cords. The extension cords were only like 50 cents a piece at Lowes/Home Depot.
Reptile Basics will cut and wire individual flexwatt pieces for you. Just make sure to let them know in the final order notice in the checkout.
They wire it for free as long as you have all the parts in your order (flex watt, cords, connector clips/insulator).
It probably comes out around a little cheaper if you solder the wires yourself. Then you wouldn't have to buy the connector clips/insulators.