It always takes a short while, but the latest version of Android is now up on the Android Open Source Project. The code comes with a list of suitable devices, with warnings for those that may experience problems or just are not supported at all. Interestingly, the Nexus 7 3G edition will not work with the open-source version of 4.2 due to licensing issues.
Unfortunately, the Motorola Xoom and Nexus S are no longer supported with 4.2, which has caused some concern with users. Apparently the Nexus 10 is the most suitable device for AOSP though, and Jeab-Baptiste Queru technical lead of AOSP, explained what this meant for the project: ““No flagship device so far has been so open, and no flagship device so far has had that level of AOSP support at launch”
You can grab the source now from AOSP, but make sure you have the right device for it.