First thing you should do is check the washer on the bottom of the pump stem.
Check to make sure the washer can move freely (it's not unusual for it to get gunked up. Try putting a flat-blade screwdriver under it and GENTLY prying upwards all around. Spray with a small amount of WD-40, and then run a few brew cycles with just water to get rid of the WD-40. In many cases, this will restore proper operation of the unit.
If not, there are a number of places online from which to purchase replacement parts for Farberware percolators. One is repairshack.com. If your percolators were made before 1998, you'll want the "old style" thermostat, part#P04-056:
^^^click^^^
If instead they were made after 1998, you'll need the new style, part#P04-303:
^^^click^^^