Look up Jeff Smith and Teskeys. Both will make a saddle of any seat size in a variety of styles.
The seat size can depend on the type of saddle because it is measured from the swells to the top of the cantle. So a saddle with a low and sloped cantle may measure larger than a saddle with an upright cantle.
I'm sure either of these companies can get what you need figured out. I personally ride a Jeff Smith Ranch Cutter, and am pretty happy with it. It was pretty broke in after about 5 rides, very comfortable.
I believe Don Rich will also make any size seat. @jgnmoose I rode in a Jeff Smith cutter for a little while. Still my favorite saddle of them all. It was a sad day when it wouldn't fit my mare anymore.
I ADORE my Continental Saddlery reining saddle, and they are hand made in Ider, Alabama. You can customize every detail and I can't imagine ever riding in another brand.
I'll also ditto the above, Jeff Smith in Terrell, TX and Teskey's in Weatherford, TX both make quality saddles that look great, hold up to use, and have a loyal following. KO Trading in Fort Worth, TX makes perfectly nice, affordable saddles but they don't have the beauty or detail of the others, IMO. I also know someone who loves her Elite Custom Saddle out of Tioga, TX and the craftsmanship is lovely.
Len Brown maybe, I have one of his roping saddles, got it decades ago. And I have one of his earlier, 1987, endurance saddles, about 1995 is when I got a roping saddle from, the then OrthoFlex. Looks like there's been some shifting around of the company, but I do believe this is Len Browns website, he makes some GORGEOUS, IMHO, saddles, they are comfortable for both rider and horse.