I want to download El capitan for my older machine but not available on App store

You can no longer download Yosemite unless of course you had downloaded it previously, then it should show up in the Purchased section of the App Store.

El Capitan is still available for download for those Macs that cannot upgrade to Sierra or High Sierra.

How to download OS X El Capitan - Apple Support

Make a note of the specifications, if your Mac can install Sierra or High Sierra then the download of El Caitan will be declined. Use a Mac that cannot upgrade further than El Capitan.

Well you will need to replace the hard drive, either do it yourself if the iMac is user serviceable or get someone to do it for you, then you will need to install the original OS from those grey install discs you kept. What was the original OS that came with the iMac? Once installed make sure any further updates have been applied via the App Store and then you can then upgrade to El Capitan.

Unfortunately, as of 2018-10-03, this no longer works because the link needs the High Sierra or better App Store, but if you're running High Sierra or better, then the App Store won't allow you to download such an old version of the macOS.

Then you will need the services of an Apple Store or an Apple-authorised engineer. You could try running the Hardware Test if you have the original install disks.

You can probably find a hard drive and installation instructions at the link below.

Other World Computing

Do you have it in your Purchases tab? It will download from there.