Finding the right horse boots for a Haflinger can be really difficult. If you are lucky and your Haflinger is petite, a pony horse boot might fit. The problem is usually not the length, but the width, which is often too small. On the other hand, a horse boot sized "thoroughbred" may fit a strong Haflinger. However, the boot might end up being too long, even if the width is fine.
There's no avoiding measuring and searching for the right fit for your individual horse. You have to measure the circumference of the fetlock and the height of the horse boot. It is definitely worth looking at equipment for quarter Horses. The horse boots offered there often fit Haflingers.