You can customise HP+30% with one stamina tonic. If you catch one of every monster in Besaid the owner of the Monster Arena will breed Stratoavis and reward you with 99 Stamina Tonics. You will get the Stamina Tonics regardless of whether you fight Stratoavis or not, it's your reward for defeating all the fiends in Besaid. Remember to have a capture weapon attached.