You might shot Tommy a conversation,he has run a muffler shop for years and might have a source. HRP http://www.jalopyjournal.com/forum/members/tommy.35/
I'm sure the OP was referring to ripple pipes not flexi ones. Like the ones for choppers and lake pipes.
In 1989 I put some Walker "turbo tubes mufflers" on my old 4x4 chevy , they were rippled like my Z/28's chambered pipes & on the boxes it read "GRRRRR! Just like the sixties!" They were anazingly LOUD! I wonder ... Do rippled pipes sound different as opposed to non rippled or is it for looks?