The Meguiar's OTC
kit contains one.
Last weekend I tried two clays by chance. When I went to clay a car, all I could find was leftover (but clean) yellow clay from the Mother's kit. A little later while looking for something, I found the blue Meguiar's professional clay (in the short round plastic jar...I use 'mild'). I went back over the areas I clayed with Mother's and was
blown away at how much the yellow clay left behind. It looked like I never did it to begin with. I can find these at Pep Boys in the bodywork section--not the car wash section--along with 105, 205, a couple of other Meg's Pro products AND low-tack green tape.
Seems like this section gets the least amount of traffic too (a quick check of the parking lot confirms this).
Of course the mild clay is going to remove more contaminants, it's also going to mar the finish. For a car that isn't heavily soiled with surface contaminants the otc kit is good enough, especially for those who don't have experience removing swirls and marred paint.