Help: Downloading a copy of Windows 7

Ok, so I just purchased a new laptop and it came with Windows 7. Since my ultrabook has no optical drive it didnt come with a backup cd. What I would like to do is download a stock copy of Windows 7 but I can not seem to find a place I can do that online. In the end I want to put this on a USB drive instead of a DVD. Any suggestions on where I can do that or if it is possible?