EDIT 27/12/19: ilMaTic BUTCH3R has managed to locate the Hidden Orb map and screenshots! Huzzah! Please refer to his solution for the weblink.
The rest of my solution is here for posterity in case any of the other points or comments help you.
[Edited on 13/12/14: The hidden orb map is no longer available so I have deleted the link. However another map is included in the screenshots. As I said before, download the images!]
If like me you started collecting the Hidden Orbs a long time ago using the Candied Skull source, but never finished this tedious job, you'll be worried to find that the website no longer exists. I was going through each of the orbs in the numbered order, which the YouTube solution cannot help you with unfortunately.
Thankfully someone else is now hosting the original map and Hidden Orb screenshots.
Hidden Orb map & screenshots (Google+ user 'Frode Knutsen'):https://plus.google.com/photos/109589836385162541954/albums/...
Frode is very kindly public sharing the screenshots, however I would suggest that you download the album on to your own computer because it takes at least six hours to get all the Orbs and this album may not be available indefinitely.
This might seem obvious, however I strongly suggest you attempt this achievement, the Agility Orbs and the Stunt Markers when all gangs have been eliminated. Do not resurrect the gangs, it will make your life hell.