Bruce: The best source I've found for B&P shells has been Natchez. They do have 28g, .410 and a few 20g loads, but some are out-of-stock. It's worth keeping an eye on the website. Shipping to AZ though is about $20/case frown I assume by pack train down the Mogollon.
https://www.natchezss.com/ammunition/shotgun-ammunition?brand=baschieri-pellagri