Running around the city is a easy way to obtain orbs. Once you reach the 200+ point, grab a helicopter and click down the to turbo speed. This will make your mini map a very large area and just start pinging () to see if any orbs are around. You can always check your progress in a certain area by going (start) > Achievements > and scroll to find "Sixth Sense."
Here are the total orbs for each area (300 total):
- Hope Springs - 87
- Green Bay - 101
- Unity Heights - 105
- Agency Island - 7
If you need help, use this interactive map: Hidden Orb Interactive Map