You can only get this after you have been to the final location of the game
You need a total of 426,000 gil to get this achievement.
There is 2 type of Theater spheres: music and movie spheres,
Once you have been to the final area, return to the airship via a save sphere and then head to Luca
From save sphere, you appear at, go south, then on the next screen go north, and keep going north until you are in the theater.
Once there you can talk to the two NPC's at the front (Short guys) the one on the right sells movie spheres and the one on the left sells music spheres.
You need to buy all 50 movie spheres from the NPC on the right. (They cost 5000 gil each)
You need to buy all 68 music spheres from the NPC on the left. (They cost 2000 gil each)
After buying all the music spheres, talk to the left guy again and he'll sell you 3 more spheres (40,000 gil each)
With all 50 movie spheres and 71 music spheres collected the achievement will pop.
Once unlocked, you can then reload the save so you are not wasting gil.