you dont need to get a parachute anywhere u automatically get 1 when u jump out of a plane
u can steal the plane at los santos airport but jumping the wall
there's also a huge 747-like jet in the Las Venturas airport at the southern most largest hanger, itll open very slowly and from my experiences the jet wont be in there til 11:55 on the game clock. but yeah, you can go rodiculously high in that thing takes awhile to climb though.