what kind are you looking for? for a gravity vent htr.or for a existing power vent?
I must be missing something here.The only question I see is were can I get a power vent kit. you guys got to be good to know the answer before the question is out.
Probably the pressure switch, or a stuck relay.
Check the hoses for obstructions and moisture.
Try tapping on the pressure switch while running on to see if stuck. Never blow into the switch, as it will surely ruin it or at least knock it off setting. If you had a digital manometer, it could be checked, but price on a new switch isn't bad.