Can I buy cocoa butter squares for cooking?
Yes, you can certainly buy cocoa butter, from many different brands. I have seen more chips than squares, but the shape doesn't matter. 

We don't do shopping recommendation for brands here, and suggesting single stores is also not looked upon well, especially since most people reading this won't live in the same place as you. So I won't send you somewhere special, I'll just say, if you can't find it in a well sorted supermarket, try in a specialty store for confiserie, and if you can't find that one, try online.