I saw a lot of them at the Gem Mall in Tucson, but I can't remember the vendor's name.
It was hard to find a P. margaratifera shell online. Here is one in NZ
http://southernpaua.co.nz/shop/index...0th8ulf0h393v5
I also found a company that makes tiles out of shell - who knew?
http://www.shelltile.com/