I only have very limited experience using crushed walnut shell, but in the research for my diy cheapo tumbler I read much as others have said, that corn cob is more of a polisher than walnut shell.  I also read that some people use brown rice as a polishing media.  Additionally lots of people mention adding novus 3 or car polish into the media.

Nino, these folk sell crushed walnut shell cheap http://www.wellwood.com.au/walshell.htm