The Fortune Sphere seems unusually rare, according to this database it can only be found if three places. That seems odd since Luck Spheres can be found more often. There has to be other places to find and win fortune spheres.
The site is correct stating there is only a few ways to obtain these spheres. The only way to obtain them by fighting is by killing Earth Eater in the Monster Arena. This feat isn't very hard when you become stronger later in the game but it's very arduous to kill this fiend quickly. Fortune Spheres are so rare because of the chance they give your characters to crit from higher luck. The game doesn't like handing these out without struggle.
I'm pretty sure there's a spot much earlier in the game where you can get one. Fairly early on, Wakka gets access to a luck node and at the time I did have a fortune sphere to activate it with. So, those two locations given can't be the only places where they are obtained.
You're correct. There is another fortune sphere that isn't listed here. It's in a chest on the Mi'ihen Highroad: Oldroad, South. You need to be riding a chocobo to get it.
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