Martin has GREAT leather headstalls....a little pricey but if you want quality leather then go for that. Also pretty much most harness leather headstalls are going to be great. They will be thick good quality leather.
All the ones I have liked the feel of have been weaver of some sort. I think it is a Westfall on my TB and I was limited on my warmblood because of size but still think it is a weaver. And I am picky as heck on my leather also!!!