It may sound like an unlikely source, but the flex pipes like the factory ones are available from
JC Whitney.
They're stainless steel and available in dipe diameters from 1.75 to 3 inch. They're on page 195 of the newest catalog (just came in the mail).
The part numbers are 47ZZ1760T for the 2.5 in. price of $52.95 ea., and part number 47ZZ1761A for the 3 in. priced at $66.95 ea.
On the web they're at
http://www.jcwhitney.com
If you need the entire pipe with the end that attaches to the manifold and welds to the cat, finding a used one might be your best bet. Good Luck.