Yuna's Whistle is a rare Instant card from Final Fantasy Commander (FIC), collector number 126. Current market price: $0.33 (foil $0.95), TCGplayer market prices via Scryfall.
Reveal cards from the top of your library until you reveal a creature card. Put that card into your hand and the rest on the bottom of your library in a random order. When you reveal a creature card this way, put X +1/+1 counters on target creature you control, where X is the mana value of that card.