I have a 17.2HH thoroughbred/ warmblooded cross that is HUGE and I just use everything normal for him. I did buy a racing bridal on ebay
for 18.00us and my normal saddle fits him fine with the largest sized girth that Stubben makes. I never knew they needed special sized tack. Everything I have is just large horse sized and everything I have bought fits him usually on the last hole or next to last but it fits :)
Try thoroughbred bridals you can get some on ebay if you look around for 18. To 21.00 they are not the finest quality but they are usable thick leather needs saddle soaping but they work fine and look great IMO.
Thee is someone who sells dressage saddles for draft horses for like 250. I know nothing about the quality tho.