BTW, is there a reason why mule is harder to find nowadays?
From what I know, mule is more or less considered as a domestic animal in Europe nowadays. That's what I've been told at any case, from a famous Greek (former) seller, whom most of us know. That's why it's difficult to find here in Europe : you wouldn't sell your dog's or cat's skin either.
I only know 2 sources by now :
one in Europe (Italy?), but I find it very expensive :
https://primordialvibespercussion.com/product-category/raw-hide-mule-skin/and Manito Percussion in the USA used to have some, but they seem to have disappeared from their site, which I just noticed :
https://manitopercussion.com/product-category/skins-and-drumheads/rolled-or-flat-skins/I'm pretty sure you could find some in Southern Europe (Spain, Portugal, Greece, Corsica, Sardinia), but you need to have some connections there, which I don't have.