Why do you want to fight only Tonberrie’s ?
Tonberries have a tendency to attack injured people. 😉
Now, I never looked for Tonberries in my FFX:I game, only in FFX. And in FFX, I just got the Tonberry (forgot the name of it… the only used commonly with the AP trick) in battle arena without having to look for them. Maybe it’s because I went around the Omega Ruins a lot. o_O
(Note: FFX = Final Fantasy X US. FFX:I = Final Fantasy X: International JPN.)