Try Traverse Town 4th District, thats where i got mine. If you dont have any other versions of Balloon like Balloonra or Balloonga, then thats why the original version isnt unlocked in the moogle shop yet.
I recommend 'Balloon' for Julius. Its faster and gives you a faster chance to dodge unlike Balloonra or Balloonga.
I beat Julius with Sora and Riku with half deck of balloon and the rest curagas.
You can buy 'Balloon' in the regular moogle shop.
*Balloon does more damage to Julius than Balloonga or Balloonra*