My friend found in Chinatown a metallic pin crest of the "Black Order", two to be more precise so I didn't have to make it. It's not exactly Lenalee's crest but since she isn't doing the Masquerade her cosplay won't be judged so I can get away with it :P One is heavier than the other so I will have to test which one she prefers on the cosplay.
However if you can't find one you can always make it using papier mache, sand it, tape a pin to the back side, and paint using a metalic silver acrylic paint. Or use plaster of paris but you will have to make a mold first.
Pin your pin where it should go and make your boxes around it using white fabric paint. Once it's dry you can remove your pin and sew on the decorative button.