Those slits/cracks probably aren't the problem. Yes they're cracks. I see those all the time. They don't seem to cause the valve to leak. Look at the seat very closely. Look for a hairline crack. Also look for an indentation on the seat.
On a rare occasion the controller will be putting out a weak signal to the valve 24 hours a day even when the controller is off causing the valve to seep water.
If you must you can buy the part at Hydroscape, Ewing, Greenmark, John Deere.
A sure fire way to determine if there's a crack in the seat is to change the entire top of the valve and leave the wire disconnected.