Do you need to have morph mastered (2) to get powersources off the monsters in Gelnika (which is the sunken plane right?)?

Or do you need to kill the enemy with the character that morphed them?

Because I tried it and I couldn't get it to work....

Weird question. Didn't quite get the jist of it but alas, i shall try. When using Morph, it doesn't automatically work. You have to get their HP down to whatever HP your Morph attacks with and then kill it with Morph, then the enemy spins around and dies if it has anything. This will be hard with those Gelnika monsters as their HP is high and hard to get down to what you want without killing it (unless you're very strong and Morph does 2000 damage, which makes it easy). So, yeah, hope that helps.

As Cloud said, you will need to weaken your enemy before you can morph them as morph greatly decreases your attack power. This will be tricky becouse sometimes you will kill enemys with normal attacks or magics before you are able to use morph.

It's easy to morph them if you use Demi 3 three or four times...

Yeah,the final hit that kills is how morph works,but,Yuffie is excellent for morphing,when she has her Conformer,she will still hit Max damage,this goes along with one of my materia combinations,Have her with Mega-All,and a Morph connected to an Added steal,and she will Steal and morph all enimies that have 9,999 HP or less.

this question has been answered but i will add my two cents. an easy place to get power sources are the rhino tank thing at the esploded reactor, where you got titan. yuffie is very useful when morphing, unless your like me and can do two to three thousand damage with morph.

<font color=red face="comic sans ms">Vincent can also do 9999HP damage with the Morph command, but you would need to get him to kill a LOT of enemies with that first, as his ultimate weapon's damage is based on the amount of enemies he's killed in the game. The Conformer is much better, as it should do 9999 every time.

Also in mt nibel you have an enemy that will be helpful for this. You have teh screamer which has only 800 health which gives you a powersource once you morph it and the Bagnarda at mt corel as well which has 450 health and gives you a guard source..see if you can morph those two and that should be really helpful with the sources...

<font color=red face="comic sans ms">Other enemies that you may want to morph for sources include the Heavy Tank at Gongaga (I don't remember what this gives though :(), the 8-eyes in the Temple of the Ancients and Battle Arena (for Magic Sources), and Spirals near Mideel (Guard Source).

Of course, the Gelnika is the most convenient place for morphing, as all the enemies there morph into a different type of source. However, the other enemies are weaker and obviously easier to kill. It's up to you. ;)

Other enemies that you may want to morph for sources include the Heavy Tank at Gongaga (I don't remember what this gives though )
The Heavy Tank at Gongaga morphs for a power source. This is the best place to get power sources as he's really easy to morph and you can fight him exclusively there, without having to worry about any other enemies.

Oh, and BTW, Morph works whether or not you have it mastered. The only effect of mastering a Morph materia is that you get another one.