Apple does not sell an optical media version of El Capitan. You need to use OS X Recovery or a USB drive to re-install El Capitan, which is relatively simple to create. All you need is an 8 GB USB drive and a computer running OS X. Instructions on how to create such an installer are here: How to make a bootable OS X 10.11 El Capitan installer drive
Making the installer drive
Connect to your Mac a properly formatted 8GB (or larger) drive, and rename the drive Untitled. (The Terminal commands I provide here
assume that the drive is named Untitled. If the drive isn’t named
Untitled, the procedure won’t work.)
Make sure the El Capitan installer (or at least a copy of it), called Install OS X El Capitan.app, is in its default location in your
main Applications folder (/Applications).
Select the text of the following Terminal command and copy it.
sudo /Applications/Install\ OS\ X\ El\ Capitan.app/Contents/Resources/createinstallmedia --volume
/Volumes/Untitled --applicationpath /Applications/Install\ OS\ X\ El\
Launch Terminal (in /Applications/Utilities).
Warning: This step will erase the destination drive or partition, so make sure that it doesn’t contain any valuable data. Paste the
copied command into Terminal and press Return.
Type your admin-level account password when prompted, and then press Return.
You may see the message “To continue we need to erase the disk at /Volumes/Untitled. If you wish to continue type (Y) then press
return:” If so, type the letter Y and then press Return. If you don't
see this message, you're already set.
The Terminal window displays createinstallmedia’s progress as a
textual representation of a progress bar: Erasing Disk: 0%... 10
percent...20 percent... and so on. You also see a list of the
program’s tasks as they occur: Copying installer files to disk... Copy
complete. Making disk bootable... Copying boot files... Copy complete.
The procedure can take as little as a couple minutes, or as long as 20
to 30 minutes, depending on how fast your Mac can copy data to the
destination drive. Once you see Copy Complete. Done., as shown in the
screenshot above, the process has finished.
Createinstallmedia will have renamed your drive from Untitled to
Install OS X El Capitan. You can rename the drive (in the Finder) if
you like—renaming it won’t prevent it from working properly.