Any automotive store like Autozone or Pep Boys carries Auto Detailing Clay. It is not the same as regular clay.
I've even seen auto detailing clay at Wal-Mart and K-Mart.
Popular brands are Meguir's, Mother's, and Clay Magic.
The clay is usually sold in a box along with a spray bottle of detailing spray.
You need a lubricant like detailing spray or a solution of water and car wash soap so that the clay glides over the paint.
The clay sort of works like a magnet in removing contaminants in your paint without abrading the paint.
I'd call the 1-800 number at Meguiar's and ask their technical support people. They will even sell to you over the phone.
You can find the number and other info at:
www.meguiars.com