I have all of quistis' Limit breaks except shockwave pulsur and I heared that you need a Dark matter where can i get this ?

Guy, you got a lot of work to do (maybe). First, your GF Siren must be at level 100. Second, you must acquire 100 curse spikes. These can be found in many places,but I think the easiest way is to go to Odin's tower and fight the forbidden monsters. They usually drop them in pairs. Lastly use the item refine ability when you have 100 to get dark matter.

<font color=red face="comic sans ms">I find a quicker way to get Curse Spikes is to fight Imps on the Esthar continent. They can come in up to groups of 3, and may drop 4 Curse Spikes each (the chance of this is increased if you equip Bahamut's Rare Item ability). You can also Mug them from Tri-Faces on the Island Closest To Heaven, although they don't show up very often. 1 Tri-Face will refine into 1 Curse Spike using Card Mod. Use Tool-RF to refine the 100 Curse Spikes into the Dark Matter. :)

Those ways are decent ways to get them...the best is to fight on the Island closest to heaven, Malboros can drop 20 at a time...and Tri-Faces can drop 8. Keep rare item OFF, they are more common than rare.

Hope that helps

All those monsters have 'm and you'll have to take long before you have 100 curse spikes though
another option is if you have the Pc version: Chocobo world !!!!! xD
With lots of luck you can get a Dark Matter from the chocobo world ;) I got it from the chocobo world.. after .... 3 times of trying ^__^ ;_;