Rees Wesson http://www.melodeons.com/ is a Castagnari dealer and could probably obtain one. Or try specialist shops such as Hobgoblin or the Music Room. I doubt any of these have them in stock but they can be ordered.

There was a discussion on melnet a couple of years ago when one sold on ebay for nearly £450. http://forum.melodeon.net/index.php?topic=9190.0

They're great fun, unexpectedly loud and certainly not toys, but that's a lot to pay imo.