It's true that Proficiency materials aren't easy to find. Flo-Joe has some nice daily exercises for vocabulary. The Cambridge website itself has lesson plans and materials for all their tests. These are aimed at teachers, and you have to register, but registration is free and the materials are perfectly usable by students already familiar with the test format. If I were you though, and if you really want to prepare by yourself, I'd just get a good self-study textbook. Good luck!

Howdy there! I don’t have any specific book or website to recommend. But I have accumulated tons of C2/C2+ materials for my students of member the years. I’d be happy to send some of it over.

For what language?

Ah....! Sorry! For English. That was not stated clearly. You're right! The Cambridge Certification is for English only I thought, but I should have mentioned it explicitly.