You can order it from your dealer, either from Vigier or from Schaller.

For earlier Excalibur Supras and Specials equipped with the 2010 trem, beware the diameter of your arm is 5 mm. On later models featuring the 2011 trem, the diameter is 6mm.

Vigier references:
Vibrato 2010 : µVIG-TIGE2010
Vibrato 2011 et 2017 : µVIG-TIGE2011
Vibrato Vigier Floyd : µVIG-TIGEVE6

Schaller reference:
6 mm tremolo arm: 13080200