This is what I use:
It is the mothers clay bar kit that I buy locally. It is the only clay I've seen locally which is why I tried it. the first kit worked well so I bought another. I think you can do a 1 series with half a bar but you have to mash it out flat and get the car clean first. They give you a spray bottle of their detailing spray to use as lubrication and I didn't use near all of it. It is handy to have around for when a bird or bug leaves you a "present".
I don't have any doubt there are better sources of clay but I find the mothers kit works fine and is readily available at a decent price.
128i Convertible, MT, Alpine White, Black Top, Taupe Leatherette, Walnut, Sport
Ordered 5/22/09, Completed 6/4/09, At Port 6/9/09, On the Georgia Highway 6/13/09, Ship Arrived Charleston 6/24/09 at 10pm, PCD 7/21/09