The seasparrow was obtained right at the start of the game,and it's on Verdant Bluffs garage
To remove it from the garage and fly it freely,push it with your body (Cars won't work) until you can fly it
How to install:Put the savefile in C:/Users/Your User/Documents/Grand Theft Auto San Andreas User Files
If you want to change savefile location,change the number at the end from 1-8