As you know, GTA isn't just about cars and trucks, we get a whole gamut of vehicles to exploit, and with that, let's clue you in on some places to look for wings.
The first place I scored a choice of two identical helis was the hospital from where I'd just been discharged. That proved to be rather random and more then not, nothing was perched on top, however, Noted during parachuting and generally driving around the full map, there are other places you can instantly take to the skies above Los Santos.
I wound up parachuting on a top secret or off-limits air field if memory serves, it's North West of Los Santos, up in the mountainous regions, I've only been there once so far, so not sure off hand, but as soon as I hit the ground I sustained terminal gunshot wounds, it's pretty hard to grab a chopper from that location, however...
To the North East is a police station in the busier side of town, should be kitty corner to the bus depot and the street urchins we saw in the trailer who camp out under a city bridge, that's South of the depot for Dachound bussing
Inside the police station, oddly, the lobby is open and permissible, as is an electric gate (located out back (to the North of the entrance inside)
Once inside, you'll find some body armor laying on the ground, away from the check in desk.
Several rooms appear to gain you entry into the heart of police activity, the hornet's nest. You'll want to opt for the door marked LOCKERS, I figured I might find a billy-club in there. What I found instead was stairs to the rooftop, and proved rather easy to evade police this time, however, once you break into any awaiting chopper on the roof, you'll already be coming under fire, elevate quickly while swerving and pitching to avoid landing gun fire on yourself. Make for the tallest building in LS, hide on the ladder system (behind it actually) and you can wait out the wanted level search. I did bail out of the heli to make that jump, but for a time you gain the police heli.
Farther on the North East section of LS, near the beach front, there's a building labeled Los Santos Government Facility. which shows up as NOOSE on your in-game map.
You can get past a single wanted star level busting through the gates (actually provoking them to open, actually) and get past and out of sight of the watchman at the gate who's your only real threat. There's again a ladder or two to the roof and awaiting helicopter. Now I don't know the real frequency they might appear, but I did these in short order, during one session, so it's very likely if you haven't had your own luck thus far.
Edited by BlackListedB, 24 September 2013 - 04:19 AM.