This article has maps etc of where you can find a helicopter: https://www.gtaall.com/gta-5/faq/27533-where-to-get-helicopter.html. I think you have already read something like this, because you are asking the same question I was. "Where are they?? They're not there!"
However, it is important to note that the helicopters do not spawn there regularly.
When I was trying to get helicopters for the Under the Bridge challenge I would keep driving back and forth between Central Hospital in South Vinewood, and the Vespucci Helipad at the Puerto Del Sol Marina. It would sometimes take me up to 4 runs between them before a Helicopter would finally spawn - and the Vespucci Helipad was the most common.
I also tried the helicopter listed at the airport, but not worth the 3 stars when it hardly ever spawns there either.
But what I think will interest you, I also tried the NOOSE HQ on the east coast and it never appeared there for me UNTIL I finished the final mission (I picked C - don't know if that matters). Now the Buzzard Helicopter spawns there ALL THE TIME.