You buy Tonics from Vendi, who is a large yellow vending machine with red lips, after completing certain challenges. In the first level, Vendi can be found just below Trowzer, who sells you new moves.
The easiest one to unlock is the extra health tonic, Wellard, which is unlocked by finding 5 Pagies. Once you collect your 5th pagie, head to Vendi and equip the tonic to receive +1 butterfly health and get the achievement!