If you are like me, you have a finite amount of hard drive space and have to delete the many ISO's you might have stored. If you were to keep ones around though, I would suggest the mac installers. I would not trust most downloads out there, and because of the certifications, you can run into issues making a boot disc or installing.
Click this to open app store to download the installer.
This will give you a fresh copy. *Your mac needs to be able to support the os for it to show up, plus some other factors I have no idea about.