You can download the image of it direct from Microsoft according to (this message on their board](http://answers.microsoft.com/en-us/windows/forum/windows_7-windows_install/where-can-i-download-windows-7-iso-i-have-a/7d964b05-2be9-4800-bc7f-3ca30356fc3d)
I realise that this is pretty much a "Link only answer" but I'm not sure how else to convey this.
Other versions are available from this post
Windows 7 Home Premium 32Bit: http://msft.digitalrivercontent.net/win/X17-58996.iso
Windows 7 Home Premium 64Bit: http://msft.digitalrivercontent.net/win/X17-58997.iso
Windows 7 Professional 32Bit: http://msft.digitalrivercontent.net/win/X17-59183.iso
Windows 7 Professional 64Bit: http://msft.digitalrivercontent.net/win/X17-59186.iso