Growing up out of town I never entred a frum store in my life until the age 14, and I was very tzniyus thanks very much! There are shells! And non jews wear skirts too! Midi skirts are very in style, I very rarely buy things in “frum” stores.
Plus money isn’t a factor, the women not being tzniyusdik have plenty of money to buy things from the mall and spend days there, plus money shouldn’t be that big of a factor in your spitituality. At least in this sense. You won’t die if you only own 5 weekdays shirts.