I used to own an own son of invitation only out of a seattle slew mare. He was 18 hands and required a large halter. I had to have one made for him specially but your horse does not seem to be near as big as him so I would think that a kathys or a blue ribbon used show halter would be your best bet. Ebay
may have that but I would look at ther actual sites too sometimes they have used ones that are traded in for descent prices.