BMSS saddles can be made up to 20", but you won't find any used larger than an 18.5". I bought mine used, and it was a HUNT for one I liked. I still ended up paying $1,000 for it, lol. Bob Marshall is one of the only quality brands that has "plus sizes". Black Forest has a "Size 3" that fits like a 17-17.5" western. Rebecca Soft Saddles can be made in any size (they hand make each one), but there's a 6-8 month wait on hers right now... Torsion also goes bigger (to 17-18" western), but they're hard to find in the USA anymore (no US Distributor any longe).
Those cheapy treeless saddles you see on eBay for $79-300 are NOT worth buying. I have seen some of those first hand, and they c.r.a.p! You're better off going bareback than buying one .