The search for red clay has come up before. I did a site search for 'red clay seed balls' and came up with these URLs.

http://forums.gardenweb.com/forums/load/cottage/msg0120442131046.html

http://forums.gardenweb.com/forums/load/permacult/msg081038047874.html

Terran

Here is a link that might be useful: Can seed balls be made from pottery grey clay?

Red clay is basically just terra cotta clay. Any local potter's supply (ceramics) should carry it. Twenty-five pound bags usually costs about $10. Try to buy it dry, rather than already having water added.

If you have a compost pile or sandy raised beds, adding some dry clay will improve the nutrient-holding capacity.

Sue

This may not be too helpful, but many areas of the country abound in red clay - in fact, I will give you all you can haul off!
:-)
Nik

heres a list

-South Carolina

-North carolina

-Oklahoma

-Alabama

and anywhere else in the south