You might consider eBay.
To know for sure you need to access the resistor and do some testing. If the resistor is bad you really need to consider the strong possibility that the resistor failed because of an aged and dragging blower motor.
With an ammeter you could test the current draw with the blower motor on the HIGH position. If the motor is pulling a large amount of current (say 20 amps or more) then the motor is on the way out also.