Make sure it is one that the trees are suited to the intended beast of burden.
We used to repair both Strathfarrar and Glenquioch type saddles for many estates all over Scotland; in my experience many, but by no means all offered for sale were made with a tree or trees ill-suited to highland ponies. Now that there are still fewer deer forests extracting with ponies you might be lucky, and find one well suited to your pony.
We used to handcut with a scythe over an acre of rye each year in order to have a supply of dry straw for the following year’s anticipated repairs.
Many of the relatively few newer saddles are filled with a type of foam, which though ‘kind’ anatomically to the back of the pony - as indeed traditional straw-filled saddles are - they tend not breathe, and to thereby make the pony ‘sweat’ excessively on and around the withers.
Edit: you could try the Ormiston family, who broke in (trained)many ponies for deer extraction and general hill work.