Do proprio criador do jogo, Jonathan Fryxelius, no BGG
As a rule, only cards that generate mana can store it, and as a follow up rule, cards use mana on themselves when using their effects, unless otherwise stated. However, if a card that doesn't generate mana uses it, it can take such mana from any card in that players tableau. So Sindra may use mana from any of her cards, and even from several cards at once, to pay for her hero ability.
https://boardgamegeek.com/thread/3423987/sindra-mana-questions