Yeah unfortunately they got rid of the digital river distro for ISO's. There are still ways, but it'll will depend on what kind of license you're using. If its a retail/Full version go here
https://www.microsoft.com/en-us/software-download/windows7
If its an academic copy go here
https://www.microsoft.com/en-us/software-download/vlacademicISO
If you have an OEM copy, it may be harder. You can try contacting Microsoft support and explain that you have an OEM key but you lost the disk. They'll likely explain you need to contact your system manufacture for a replacement. But explain you have a system builder license and need the media and I've heard of people getting disks at the cost of delivery and a small surcharge. You can find them at
https://support.microsoft.com/en-us/contactus#
If you have an OEM license and nothing above works, you could someone with a Dell or something with the install media and go from there. Most OEM OS disks tend to be the same thing.
Finally, you could just give Windows 10 a try. You can use a Win7 or Win8 key with a clean install of Win10 and it'll work.