Master Sphere

[<Sephiroth>]

Hi :)
Can anyone tell me where I can get more Master Spheres?

[ReloadPsi]

Sometimes Dark Aeons drop them upon defeat, but beyond that I think they're only available as a one-off prize for defeating Nemesis.

[<Sephiroth>]

Aha, I see...
But to get Nemesis, I have to capture 10 of all monsters right?
I have a problem finding Tonberrys in the sunken cave... Are there special parts of the cavern they are located?

[ReloadPsi]

Heh, I had the same problem: Tonberries were by far the very last thing I ever successfully finished capturing.

Skimming GameFAQs I found this in the Monster Arena guide:

Arcaneja and others on the GameFAQs FFX board have reported that the Tonberry
rate was highest in the "green" room to the right of the first 4-way
intersection

Wish I'd known that >=(