crossman 2240 valve stem seal

Hey guys my 2240 started leaking from the breach and barrel. I took the airgun apart and found that the seal on the valve stem is damaged. Is there a hack for fixing this, or a good place to get parts from? I chucked up the valve stem in my cordless drill and spun it on some 220 grit sand paper trying to buff/sand the damage away. But, it still leaks.