It could be 1 of 3 things.
1. The fan relay in the board could be bad: Need to check for voltage at that point
2. The capacitor for the motor could be bad: Measure MFD of the capacitor
3. The fan motor could be bad: If you have voltage from board and capacitor is good, motor is bad.
You need a multimeter to check voltage and get MFD reading; picture of capacitor is included.
No they are not.
For the board, you will need to know model # XXXXX furnace or part # XXXXX board and can probably find it online. The capacitor and blower motor can be found at a local supply house or online.
Board cost is 150 - 175
Cap is 10
Motor is 60 - 75