How can I install Windows 7 on a Retina MBP with no optical drive?
Here are the 32 bit and 64 bit ISO files that you would download for Windows 7 since Microsoft only supports digital downloads of Windows 8.
You could also look at this Windows 7 forum for more download links.

You would use Disk Utility to restore the ISO file to a USB Flash Drive (4 GB) (See picture below, but ignore the error :) )


In Boot Camp Assistant, you would then select this ISO file in the USB Flash Drive, assign how much space you would like, and it should complete it for you.