In the Gelnika you can find Yuffie’s ultimate weapon, Conformer. This is very useful for morphing; at a high enough level this will cause 9999HP damage per hit with the Morph command. Once you have the Conformer, you should be able to get sources fairly easily.
The enemies morph into these sources:
Unknown ~ Power Source
Unknown 2 ~ Guard Source
Unknown 3 ~ Magic Source
Serpent ~ Mind Source
Bad Rap ~ Luck Source
Poodler ~ Speed Source
Easy. 😀