If it is legal in your state then almost anything that has both CS and OC. Be sure it at least has OC or it is just a gimmick.
Also be sure to train them to use their thumb as their "Trigger Finger" as oppossed to their index finger. Better targeting control when using your thumb. That is unless of course it has a trigger config like a firearm instead of a "button" trigger.
jalcon is correct Fox Labs is good. I carry Fox Five Point Three while on duty.