I have one main question about this. where do i get it?

Your HP caps at 9999 and cannot go further.

So their is no such thing as an hp limit break materia? or is it like a cheat with gameshark. beacause ive seen my buddy has some and he dont remember how he got them

You can get disabled materia if you use a gameshark. But they won't have any names and will appear blank. You'll just see the coloured circle. I haven't used them myself though.

What do they do?