While its true as NewArt says that used catalytic converters 'are perfectly acceptable" as a fix to a dead cat, California prohibits their installation. Here's a quote from the California Air Resources Board website:
"Information About New Regulations for Aftermarket Catalytic Converters
The ARB adopted revised regulations and evaluation procedures in October 2007 for the approval of new aftermarket catalytic converters sold in California. The requirements apply to all new aftermarket converters produced for sale or sold in California after January 1, 2009. The requirements also sunset provisions allowing the sale of certified used catalytic converters beginning July 10, 2008. This means that no used converter can be legally advertised for sale, sold, or installed in California after this date"