There was an NPR story that dug way into this myth. And while they weren't able to track down any actual suppliers of pig bung, they did make their own pig bung

There was an NPR story that dug way into this myth. And while they weren't able to track down any actual suppliers of pig bung, they did make their own pig bung Calimari supplied by a local butcher. IIRC, the blind taste testers all preferred the pig bung over the real deal, and they all considered themselves to be Calimari "snobs."