I probably have several hundred hours invested in Single-Player, as I don't play GTAO...I hope this will lend me a little credibility here:
(these all apply to Single-Player unless noted)
Unarmed Buzzard can *sometimes* be found on top of the police station next to Franklin's tow-truck yard.
Cargo plane can be "put" in your hangar using a game-save editor, but it explodes when you go near it. A workaround might be available. This plane (Model Name: "Cargo Plane") doesn't spawn anywhere in free-roam, and only appears in the mission "Minor Turbulence". Some of you are confusing this with the Boeing 747 (Model Name: "Jet"), which is smaller, and has row(s) of windows down the sides. (Not sure if it's one or two, real one has a row and a half.)
Regular Asea: Never seen this spawned outside of one mission - it's driven by the wife of the guy
. It can be obtained by killing her before or after she drives off in it. I kept and modded it just because it's not otherwise obtainable. Not sure if others spawn when I'm driving it. Checking now. (So far no, just seems to help spawn Dilletantes.) Not sure why people are saying they spawn - unless it's country-specific? I've done 40+ storyline playthroughs, never saw it once. Some could be mistaking this for another vehicle.
Skylift can be flown after the ending of one of the heist missions. It might be possible to save it at someone's purchased helipads. I never tried it since the magnet underneath doesnt work on anything after the end of the mission.
Dinghy (misspelled in original post - a "dingy" is another name for...well, y'know.): Not sure why this is listed unless the original poster had not yet purchased the dock, and found the Stranger/Freak mission that appears there.
Prison Bus does spawn inside the prison. Not sure if enough gates open to allow you to exit, apparently so according to another post in this thread. I believe two of them spawn side by side.
Annihilator: Someone said this spawns at LS International. It doesn't. It doesn't spawn anywhere in single-player, but does spawn in several locations in GTA Online once you reach a certain rank, or when you're riding with another player at or above that rank. But again, appears nowhere in single player, except maybe on TV.
Merryweather Mesa: Can be obtained at the end of the mission "The Wrap Up", by choosing one of them to drive off with. It IS possible to save this vehicle after you meet up with Trevor. I dropped it off at Michael's garage after the mission finished. Its never a good idea to save a vehicle to any garage before a mission is finished. Rockstar supposedly fixed that, but I dont take any chances. The Merryweather Mesa can also be snatched up during the mission "Meltdown", but will require you to fail the mission. Note: The Vacca (also a non-spawning vehicle) is also obtainable in both of these missions, but with less urgency, since it can be purchased online.