I think the best way to get Curse Spikes is to mug Tri-Faces. I think you get a few curse spikes per enemy. I am not sure if that's right or not. If you have Diablos, equip Mug to Squall and use it during battle. You can find them before you encounter Edea at Galbadia Garden. I hate fighting them, they annoy me! I think you can also find them on the Island Closest to Hell and the Deep Research Centre. I hope that helps. Good luck. You could also modify Tri-Face cards but you only get one per card. It would be better off trying to Mug them.