You can not get Diablos at Ultimecia's castle. The only GFs you can get there are Cerberus, Siren, Carbuncle, Leviathan, Pandemona, Alexander, and Eden. The GFs that can be drawn from bosses and Cerberus. That's all: No others. So if you can't get the Magic Lamp from Cid, you're out of luck, because there is no other way to acquire Diablos.