If you want new, I think Thorowgood or Wintec would be the best bets. The challenge is that they simply do not work for all horses. If you could stretch your budget to ~$1500, I think you’d have a bit more choice on the used market. In today’s economy, who know how low an offer people would take. An 18" seat is not that hard to find. Are you in the US? If so, I would also look at:
https://www.aikentackexchange.com/
https://shop.olsonstack.com/
If you are new to saddle fitting, this is a helpful resource (and remote service):
https://www.thesaddlegeek.com/english-saddle-fitting-in-one-hour-what-to-watch-and-read-first/