Max-Torque makes them in 10, 11, &12 tooth for the #41, #420 or #40 chain. You can order them through BMI or Comet Kart Sales. Just spec out 5/8 and that you want the black spring.
The standard off the shelf clutch is 10 tooth but Max-Torque makes the 11 and 12 tooth in the # 41 chain. You just have to ask them to order one for you. We make them all the time but they are mainly drop shipped to the end user, they don't like to stock slow moving items.