While I don't disagree with what others have said about thrift stores, such as Goodwill, their problem is that they're not very good about keeping their clothes in the right size categories. You never know what condition they're in. You also don't know the vintage. On the other hand, that can be an advantage in finding something from an earlier era that appeals to you. I have also found some insanely great bargains. In larger cities, Goodwill has boutiques where the better items are sent at prices closer to resale prices than thrift stores. On the other hand, a silk, Pendleton dress in excellent/like new condition for $40-50 is still a screaming deal! And they will let you try things on. Most have unisex fitting rooms.
Off-price stores, such as Ross Dress for Less or TJ Maxx, are great places to find new, up-to-date, brand-name items at resale store, or less, prices. A large percentage of my oversized wardrobe came out of those stores. Partly because I figure anything for $10 or $20 that's at all appealing or different is worth the chance. Anything over $40 has got to really jump off the rack and tell me I can't live without it! As I said, I've taken a LOT of their merchandise off their hands.
Department stores are very discriminatory: the only color that interests them is GREEN (money). They've almost always got something on sale. Their associates will gladly help you find whatever you're looking for. And they'd rather you try it on in the store and get the size and look right than buy it and return it.
Lastly, there are resale and consignment sites online as well as the online sites of all of the major department stores. What you have to keep in mind is that, especially with the resale sites, you could easily pay double the shipping fee for the right to try on the item. Women's sizes changed a year or two ago so, for instance, what was a 14 in the old system is now a 12. Better yet, there is also the phenomenon known as vanity sizing, where the manufacturer purposely puts a lower size on the item so the buyer can fool themselves into thinking they're thinner than they really are.
Finally, there is the eccentricities of design. Maybe you've found a size in a label that fits you perfectly so you pick an item of that brand with a very different design to it and you can't even get into it! Blame it on the cut. This is more likely to be an issue with dresses and I know you only want to wear skirts for their practicality, but once you venture into a new neighborhood, you never know how far you'll go!
Good luck and have fun!
David, the PDX Fashion Pioneer
Social norms aren't changed by Congress or Parliament; they're changed by a sufficient number of people ignoring the existing ones and publicly practicing new ones.