Considering you need the disk, you're reinstalling, correct?
In that case, the suggestion to go to Windows 10 isn't a bad one (assuming your hardware is compatible of course) - use your W7 key to install.
Otherwise, head to the link USAFRet posted, input your key and download the ISO.
I don't know if the W7 option gives the "creation tool", so you may have to make your own disk/USB.
You don't simply drag & drop it onto a USB, I want to make that clear. You need to create a bootable USB.
There are numerous tools available online to accomplish that (assuming MS doesn't provide the media creation tool for W7, which'll create a USB for you). Personally I use Rufus. Never had any issues with it. https://rufus.akeo.ie/