While Techworm is a reputable tech blog, is superior because it downloads the file directly from Microsoft's own secure content delivery network, ensuring you do not get a corrupted or infected file. Only use third-party links if the official Microsoft workaround stops working.
(Search for Microsoft’s official MSDN checksums to compare.)
If you're unable to find a reliable source for a Windows 7 ISO file or prefer not to download it from a third-party website: