Question: Unable to download El Capitan to create bootable USB drive
I have a mid 2009 MacBook Pro, and recently something occurred to the hard drive to the point that it will not boot (it gets stuck about 1/3 of the way through the progress bar). I have booted in to Disk Utility and attempted to repair the drive, but I get a message that says it cannot be repaired. I have a separate iMac running Sierra, and so I created a bootable USB drive with High Sierra, and also got a new hard drive. After multiple attempts and failures to install High Sierra (I keep getting the prohibitory symbol), I did more research and discovered that my vintage of MBP does not support Sierra/High Sierra (only mid 2010 models). I attempted to download the El Capitan installer on my iMac from the App Store, but it won't download there because it's running a newer copy (Sierra). Since I can't even get to my MBP to download it, how do I get a copy of the El Capitan installer so I can create a bootable USB drive?
iMac (27-inch, Late 2012), Mac OS X (10.0.x)