There isn't any place that we can recommend.
With your Ultimate, did it come with the license/product key?
And we don't want to see it!
How to See Your Windows 7 Licence Information
https://www.sevenforums.com/tutorials/19921-license-information-windows-7-a.html
List of Activation error codes and what to do about them:
Error message when you try to activate Windows 7 or Windows Vista: "An error has occurred"
Contact Microsoft:
Activate Windows 7 or Windows 8.1 - Windows Help - Microsoft Support
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If you had come here when you had the problem years ago, you could have reinstalled what came preinstalled on it, 7 Pro
Read through this as you say you could not get the required ISO years ago.
You can borrow the exact same edition of Windows 7 Pro Retail DVD and use your Product Key on the COA sticker to reinstall the operating system.
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Or you can use this for preinstalled OEM or Retail Licenses. This is on a first come, first served monthly basis.
Download the correct ISO file and make your own disk and use the Product Key that is on the COA Sticker.
https://www.heidoc.net/joomla/technology-science/microsoft/67-microsoft-windows-and-office-iso-download-tool
Download: Windows ISO Downloader.exe
And here are instructions on how to burn ISO images to disk:
http://www.snapfiles.com/get/ISORecorder.html
You change the Boot Order in the BIOS to be able to boot from your installation media:
How to Boot Your Computer From a Disc or USB Drive
How to access Boot Menu:
https://neosmart.net/wiki/boot-usb-drive/
Then do a clean install:
http://www.sevenforums.com/tutorials/1649-clean-install-windows-7-a.html
The COA sticker is either on the underside of the Laptop or in the Battery Compartment.
With a Desktop it is either on the side, rear or top of the Computer Case.

Using the downloaded ISO file Method may require phone Activation.
It may require that you use phone Activation.
"How to Activate Windows 7 by Phone"
http://www.sevenforums.com/tutorials/18715-activate-windows-7-phone.html
The above method also will require you to go to your Computer Manufacturer's website > Support and Downloads Section > locate your Model Number and Operating System > then download and install the latest Drivers for it.
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Using the USB method in conjunction with the above Windows 7 download:
http://www.sevenforums.com/tutorials/2432-usb-windows-7-installation-key-drive-create.html
"How to Get and Use the Windows 7 USB/DVD Download Tool"
http://www.sevenforums.com/tutorials/31541-windows-7-usb-dvd-download-tool.html
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And this is very good and easy to use:
Rufus - Create bootable USB drives the easy way
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Important: Read about changes to how Windows 7 obtains Updates after a clean install until end of Support in Jan, 2020.
https://support.microsoft.com/en-us/help/4569557/windows-update-sha-1-based-endpoints-discontinued