I've been looking all over the crater for these things. I can't seem to get them in a random encounter. Master Tonberry is only found in one small part of the crater so I thought these might also be.
Can somebody please tell me where to battle them?

Yes. Go to the junction where you tell your party to split up to go left and right and take the left path (towards the bottom of the screen). Now you'll see a cave to the right centre of the screen - enter it. Voila! You'll know you are in the right place because the music has changed, and the area looks a lot different. Magic Pots are frequent but you will face other unwanted enemies too.

Whoah, I thought someone wanted some weed down here. I guess not.

Thanks for the info... Not you Jarosik!