High Strung by Fun Inc
You bring out a card case and an envelope with a "prize" on it. Upon removing the cards from the case and spreading them, each one is seen to have a string attached to it. You then replace the cards in the case, leaving the strings draped outside of it, and talk about an old carnival game in which a player pulled a string and won whatever prize was on the attached card.
Offering the spectator a chance to play, a spectator selects and pulls a string. Then, miraculously, the selected card matches the prize card in the envelope - your spectator is a winner!
Comes complete with special Bicycle deck and printed instructions.