Auto-Life is in the upper-lefthand corner of the Sphere Grid (
http://db.gamefaqs.com/console/ps2/file/final_fantasy_x_sphere_na.gif). I think that's where Yuna starts out, but you need a Level 4 Key Sphere to unlock it. You can win those from Omega Weapon, Seymour Flux, Spectral Keeper, Land Worm, steal them from Nemesis, and bribe them from Chimera Brain. You also find them normally.