Is your gun a SMALL framed Witness (possible, if you bought it used), or the larger frame?
If it's the larger framed, you can buy one directly from EAA.
If it's the smaller frame, you may have a wait as you search for a used one. The CZ units (Kadet Kits) will NOT work with either of the Witness frame sizes. (EAA may have some available, but I doubt it.)
They all work well, and are a hoot to shoot.
(I have had a bunch of CZs and use the Kadet Kit on my 85 Combat; it's one of the most accurate .22s I've owned.)