A synthetic might be a good idea, if you want to keep costs down. Abetta, Fabtron, and Big Horn synthetics are all quality USA made saddles.
If you don't mind spending a little, then I'd recommend a Bob Marshall trail or endurance saddle (treeless, you need a specialized Skito pad
, no horn on the endurance model), or a Circle Y trail saddle (any of their models are nice), or ... ? There are a lot of good brands out there, you just have to pick the one that fits your horse and fits your needs best.
You want something that is made in the USA, preferably a recognizable brand, or from a company that has a reasonable trial or return policy. If it has a regular tree (not a flex tree) you want ralid or HARDwood. Stay AWAY from fiberglass or fiberflex trees. They are a sign of a low quality saddle.
You can find many good saddles used. If you're on a budget and want a leather saddle, used is the way to go. Don't pass over OLD vintage saddles either. Those oldies are often great finds.
You want a saddle 1.5-2" smaller than your English measurement. Most women take 14.5-15.5" seat size. If you have an ample rear, then a 16-17" seat might be best. I take an 18.5-19" English saddle and I like 17" western saddle. My husband takes an 18" English and he likes 16-16.5" saddles.
The type of seat will also effect the size you need. If it's over padded (like Tuckers), then you'll need to go up a size or size and a half. I take an 18.5" in Tuckers. If the seat is flat, like a cutting saddle, you might be able to fit in a smaller size. If the seat is really sloped in the front, with a deep "pocket" for your rear, you might need a bit bigger saddle. The best way to decide is to sit or ride in a few different types of saddle and see which one you like best.
The tree size goes by the "bar" angle/type. If your horse takes a Medium or Regular English saddle, then Semi-QH bars are probably the way to go. If your horse takes a medium-wide to wide tree then QH or Full QH bars might be best. If your horse takes a wide to x-wide tree, then Full QH bars are likely a good fit. Again, you need to try the saddle(s) on your horse to find out. Just like English saddles, most western saddle manufacturers will vary in how they make their trees. Generally, once you find a brand and size that fits you both, you should be able to stick with that brand, usually, lol.
Good luck saddle shopping! If you can let us know which seat and tree size you think you need, we can look around and give you some suggestions.