Are the sources used to support Diamond's argument in Guns, Germs, and Steel valid?
The sources used in this book are absolutely valid. There is no real way to argue that they are not. Diamond uses articles from well-respected journals such as Science and books published by reputable academic publishers such as the University of Chicago. These sources can be found in the "Further Readings" section at the back of the book.
While we cannot argue with the validity of Diamond's sources, we can argue with the validity of the conclusions he draws from those sources. For example, Chapter 2 draws on a number of books about Polynesian societies. The books are reputable. Diamond uses them to argue that the differences in Polynesian societies were caused by geography and nothing else. This is his own conclusion and people may wish to argue with it.
Overall, Diamond's sources are indisputably valid. What can be disputed is the way in which he uses those sources.