On July 14th, 2010 FarmVille introduced Mystery Game and to pop balloons you use Mystery Game Darts.
It always can be bought from market with different price and amount of Mystery Game Dart.
Pack of 3 cost 57 cash.
Pack of 5 cost 90 cash.
Pack of 10 cost 160 cash.
It was also a reward for finishing different quests (those darts are in Gift Box) or you can get free trow from FarmVille from time to time. Unlike quest darts you can not save up FarmVille free trows, so you can have only 1 free trow!