You can download the Mac OS X El Capitan on Virtualbox on Windows by accessing the link below. Also, make sure that you have enough back up before you proceed with the same. There are a lot of duplicate ISO images on the market and it is best to download the same from a trusted source. The OS X El Capitan v10.11.6 Update improves the stability, compatibility, and security of your Mac, and is recommended for all users. This update: Resolves an issue that may prevent settings from being saved in accounts with parental controls enabled.
Depending on the MacBookPro you have you may still be able to get El Capitan, but if the App Store recognises that your Mac can upgrade beyond El Capitan you will be denied it.
So first of all click on this link.
Then name it El Capitan Type and Version = MAC OS X (64bit) and click on Next. Allocate more than 3GB of ram and click on next. Then click on Use an existing virtual hard disk file and browse for the virtual hard disk file extracted from El Capitan Disk ISO Image and click on Create. Then select El Capitan virtual machine and click on Settings.
Look at the compatibility check, scroll down to section 4 and click on Get El Capitan.
now if you can't download El Capitan
Download Os X El Capitan Dmg
then you may be able to upgrade to Sierra or High Sierra, again click on the links below.
If your MacBookPro is young enough it may be able to upgrade to the latest OS Mojave,
El Capitan Iso Image Download
Jan 17, 2019 2:38 AM