If you have an older Mac running an earlier version of macOS, you can upgrade to El Capitan directly from the Mac App Store, if available. However, if you're looking for an ISO, here are a few alternatives:

hdiutil attach /tmp/ElCapitan.cdr.dmg -noverify -mountpoint /Volumes/install_build createinstallmedia (Apple’s official tool) to copy the installer files:

Apple provides OS X El Capitan completely free of charge. The safest and most reliable method is to use their official links.

El Capitan is forgiving but requires specific hardware to run smoothly: Requirement Recommended Intel Core 2 Duo (64-bit) Intel Core i3 or better Memory (RAM) Storage Space 20 GB+ (SSD highly recommended) Current OS OS X 10.6.8 or later