darkstar,

i got mine at walmart.. but i really had to search for it. it was the only one and it was on the very top shelf. it was nine something. it has a little picture of a blue clay man on it.. the brand is Clay Magic.

I think the box said you have to do it on an unheated surface, so either in the morning or in the shade i guess. And I don't think the outside temperature really matters... because before you apply the clay you spray this "lube" stuff onto a section of your X (they say do 3ft by 3ft at a time).

And yes, its probably a good time to fix nicks after you do it because it takes everything off of your surface.

Have fun!