I know Abetta and Circle Y does make flex tree saddles. I also heard Abetta considers to be the best out of synthetic, but... I tried 3 different Abettas and my butt like neither of them. Somehow my legs go too forward on it and don't feel good. And my qh didn't like it much when I borrowed it from friend of mine to try (didn't even try on my paint).
I know Chick Saddlery. :) In fact I'v been there this past Sat: they have lots of saddles and good prices. And they were VERY patient with me trying saddles (i'm sure guy wanted to kill me in the end lol!). I do suspect most of vendors in my area get saddles from there and add their extra 50 - 100 - 200 $$. I asked about flex-tree and he said he never had complaints with it. I was still thinking about Abetta, but he checked the measurements I brought with me (shape of my horses withers), and Abetta is too low for my paint - will push on withers. May be make sense to call them and ask for advice?
I'd be very happy to recommend the custom-made, but it's out of my price range, so don't know anyone. :(