I have used this one. Works well, I have also used a generic one, saw no difference http://wizardsproducts.com/mystic-clay-120g/
I use Mcguires as I can get them locally from Auto Zone without paying shipping. How much difference can there be in a clay bar?
Chit i swiped some from the kids, that they had from the craft store -n- it worked just fine. Blue,pink,green all kinds kolors, they all did the same for me. Part of the trick is WET-slippery. OR as above both work well....