Litterature :

Editions Magellanes : Very interesting publications about Cetoniidae (Cetoniimania, of course !).
Insecta : An online bookstore with very interesting old (and new !) papers
Kabourek : Here, you can find field equipement AND litterature !
Paul Schoolmeesters' website : All the publications about Scarabaeoidea, many links to original papers. An amazing website !
Pemberley Books : THE historical online bookstore (Natural History, Entomology, ...)

Friends :

Colepido19 : Interesting galleries about French and Malagasian Cetoniidae, with many rare  species
Jean-Louis Alpansèque : Many very nice fotos of Palearctic Cetoniinae
Noël Mal : A beautiful Flickr gallery with many beautiful pictures of Cetoniidae : The very nice website of Guido Sabatinelli

Forums :

Entomology Italiani : Many very interesting threads and very nice fotos
Le Monde des Insectes : The French forum, with an amazing photo gallery

The Insects Collectors' forum : The best international forum !