Adobe officially hosted a free trial of CS6 for years. If you can find the official trial executable (from Adobe's former download.adobe.com domain), combined with a legal serial number you own, this is the safest method.
Adobe has authorized certain resellers (like ProDesign Tools) to host legal "Legacy Installers." While they don't provide public "Index Of" access, they offer direct download links for users with valid serial numbers. Index Of Adobe Photoshop Cs6