Not until you assume control of the airship can you travel to a place called the Omega Ruins. A lot of unique spheres are located there but not until the last quarter of the game.

And if they're not there, they're nowhere. Items are practically THE most few things you can find in FFX. And that made me mad. The only way to get something is to go through some bogus sidequest where the guy you challenged gives it to you, run around 10 different areas to get it, or live with the fact that you can't get it and simply walk away. Luck is good, 255 luck is better, but from my experience, luck stats to max isn't possible in FFX. There's just not enough...STUFF. It takes too much...TIME. The game refuses to give out rare items unless you go through the special details. Whoever said that FFX gives you more opportunity to customize than FF8, well, I'm having doubts on your philosophies.