You will find the file named like Adobe Photoshop CC 2015 v16.0.1 Portable.7z or .rar on various archive sites. Verify file hashes if possible (look for comments from trusted uploaders).
For genuine use on an older system, you can install the official CC 2015 version through the Adobe Creative Cloud Desktop App by selecting "Other Versions" under the program menu. Key Features of CC 2015 Adobe Photoshop CC 2015 16.0.1 Portable