For me the Cuban 800 always spawns at that small airstrip for the drug trafficking, sometimes only the one to trigger the mission and sometimes another one which is parked. Also at the Sandy Shores airport if you wait around for a while you'll see random AI aircraft land. It usually rotates between the Cuban 800, that stunt plane, and I haven't waited around long enough to see if the Velum shows up. So just keep looking around and I'm sure it'll show up eventually.
Is there seriously no way to get into LSX without being wanted? In San Andreas this would happen until you got a pilots license, which I was under the impression Trevor already owned, and even after flight school with Trevor it still gets me a wanted level. I managed to steal one of the 747s but it's a pain in the arse trying to manouvre between police cars while accelerating down the runway. And of course if you want to land the thing again LSX is your only option so you will get an immediate wanted level upon returning. It would be nice to just be able to fly and come back without having to die or be chased all around the airport. I can understand the other two attracting attention but I don't think it should apply to Trevor.