Thrift stores are my personal favorite place to shop, due to the variety of things you can find. The good thing about thrift stores is by purchasing from them, you aren't creating anymore demand for the creation of clothes themselves. Some people who are cruelty free in their shoping habits are fine with buying animal products such as leather from thrift stores because of this. You can find quality items for cheap at thrift stores, you just need to remained focused and know what you're looking for. Click here for some tricks & tips for thrifting.
Vetta is a brand I have personally purchase from before. They have amazing quality pieces, most of which you can wear multiple ways. That's right, they're convertible! Vetta creates pieces in "capsules." They create 5 pieces that go together in each release that can be worn multiple ways. I wholeheartedly believe in Vetta's vision.
Vetta is sustainable in the way they think about their pieces, their factories, and their shipping practices. They have a section of their website devoted to their sustainability. Read more here.
I haven't purchased from Everlane before, but I've heard great things about their sustainability practices and quality. Everlane sells the basics. When I think of them, I think solid neutral colored pieces that are high quality.
They have evironmental iniatives such as a carbon commitment, a goal to eliminate all plastic from their facilities and shops by 2021, they also only use certified organic cotton and clean denim. Their website appears very transparent about their costs. Read more here.