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

For earlier Excalibur Supras and Specials equipped with the 2010 vibrato, beware the diameter of your arm is 5 mm and is not available anymore, you have to replace the bar insert that goes into the base plate to use a 6mm bar, but this has to be done by a luthier or mechanical.
On later models featuring the 2011 trem, the diameter is 6mm.

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

Schaller reference:
6 mm tremolo arm: 13080200