As has been stated in this thread, AOSP is the source code which can be used to build ROM's.
That said, if you're looking for the stock factory images that come on the Nexus 5, those are available to download here:
https://developers.google.com/android/nexus/images
keep in mind, however, that the factory images are not .zip's to be flashed in a custom recovery. Rather, you need to use fastboot to flash the factory images. but it's easy, and it'll get you back to 100% stock. (although not 100% AOSP).