There are thousands of clothing retailers in the world. I am so surprised that there isn't one internet store that caters to women , who are not of your average run way model's dimensions. Ya know, like a real person. I really find it hard to believe that marketers haven't found this sea of customers.
Having spent decades in the apparel manufacturing industry, I can say without hesitation that human bodies come in far more shapes and sizes than most people imagine. Ready-to-wear manufacturers stay in business by selling to as many of those people as possible. Like every other industry it comes down to numbers. You gear for "average" humans with the hope that most can find something in your size range that works.
Outside of the highest end of haute couture, very few manufacturers are so foolish as to pitch their offerings to fashion model dimensions. Those that do are out of business lickety-split. Average size for US women is a generously proportioned 16.
Most brick & mortar stores purchase on a bell curve with 16 at the apex, so in a given style they might buy 1 piece in size 6, 2 in size 8, 3 in size 10, 4 in size 12, 4 in 14, 6 in size 16, 4 in size 18 for a total of 24 pieces. Usually sizes above 18 move into plus size or "women's" sizes rather than "missy." When a store is out of your size it's usually not because the store didn't buy it but rather because they ran out.
While making patterns to their size specifications, manufacturers use mannequins and live fit models (standard fit models wear size 10, not size 0, by the way) to see how their clothing will fit and hang, then tinker accordingly. It's both art and science.
I happened to be a size 10 for almost all my adult life, and though I wasn't proportioned properly to be a fit model, I was often asked to try clothing on and give feedback as to style, comfort and fit. As were virtually all the other female employees who wore size 10. Our advice was taken seriously so we were happy to help.
Generally speaking, the further outside of average proportion your body is, the more difficult it will be to find ready-to-wear that fits well. That's when people learn to be creative and employ some of the strategies mentioned above.
Hope this quick primer was helpful to some.