Historically, Microsoft hosted digital river links for these files, but most of those official links have been taken down.
The only remaining legal source for a genuine Windows 7 Professional ISO is a Microsoft-issued product key that can still activate the OS (e.g., via the Media Creation Tool archive on Microsoft’s software download page for older versions). Verified SHA-1 hashes for official releases (e.g., en_windows_7_professional_with_sp1_x64_dvd_u_676939.iso ) are publicly documented. Any ISO mismatching these hashes should be considered tampered.