link removed just in case 
 
if you dare :)
 
i have the original direct from microsoft stored away, i'm not moving from 1511 until if and when it is necessary. updates have been disabled for months now
 
i think you can still download it from microsoft, you just have to know where to look

1511 running great here on two PC's.  I deleted the 1511 installer from a flash drive that I needed for another use.  Good thing I take regular image backups, so not likely I'll need another clean install. 
 
But I looked for an official link for the 1511 download from Microsoft as a suggestion to an earlier comment posted here and ran into a dead-end.  Not gonna risk a torrent or unofficial mirror, no thanks!

i just downloaded direct from microsoft to check, and you can still get it, it's the same as the version and build number i am currently running, which i haven't allowed to update for months. so you can still get it. they have made it hard to find to just download it, taken away references etc, but it is still there.
 
the link (non microsoft) that i originally had put up seems to be legit, md5 sums etc match, but i don't blame anyone who wouldn't want to download from a non official site, but you don't have to.
 
1511 works like a dream, have it on 3 desktops and 2 laptops, no issues whatsoever, no intention of moving for the foreseeable future