It may well be that Italian-speakers know of other online sources-- and feel free to email me with them if you do-- but here are two that I can vouch for:

• Internet Bookshop Italia: The iBS is like Amazon.com or Chapters/Indigo.ca in its look and feel, and they carry lots of Rizzoli's (Guareschi's publisher's) titles. You order with a credit card, and yes, shipping to the US is expensive. BUT, if what you need is available anywhere, it's likely to be here. Search on "Guareschi" to see what's in stock; secondary sources about GG may also appear among the search returns. This site is 100% in Italian, but because it works like other, similar sites, even I (with no Italian) have successfully ordered from it in the past.

• Barnes and Noble: Yes, I know they're an American purveyor of books in English, but they are also a New York business, and if you want your Guareschi in Italian, they can get it for you! Search on "Guareschi" and you'll find plenty of Italian titles, not all of them about Don Camillo, in the list. I don't know whether every GG book in print in Italian is available through B & N, but if you're in North America, you might as well check here first and hope to save on shipping.