Before searching for third-party ISO mirrors, you should always try the official Apple route. Apple still provides DMG files for older operating systems via their support pages.
Despite being several years old, El Capitan remains a vital operating system for specific use cases: Mac Os X 10.11 El Capitan Iso Download
If you need a hand with the or if you're hitting an error during the installation process, let me know! I can help you troubleshoot specific boot errors or VM settings. Before searching for third-party ISO mirrors, you should