When I'm not taking clothing donations from awesome and generous friends, I prefer to get my clothes from thrift and resale shops.
Two reasons: 1) My income is well below the poverty line 2) Reuse, Reduce and Recycle.
Buying second-hand apparel is not only economically feasible, but environmentally friendly. Plus, you learn to develop your own unique fashion sense since there is no "look of the season" at a resale shop.
My favorite is Alternative Earth. It's basic, comfortable, fashionable, sustainable* and--boom--affordable! Look at this beauty below for only $12.99 ... look at it, I said!

You'll notice that all the clothes seem ridiculously comfortable. They come in classy earth tones with clean-cut lines you can dress up or down, depending on your accessories.
Finally, the apparel isn't only cute and fashion-forward, but it's good quality. Stores like Forever 21 and H&M might fool you into thinking your saving big dollas' (and looking tight while doing it!), but put this in the ol' washer a couple of times and you start looking more frump, less style. I know this from experience.
Website like Greenloop are amazing resources. They can help you, me and everyone we know make responsible, sustainable and eco-friendly choices when it comes to fashion. Awesomeness!
*materials: "organic cotton, recycled polyester and man-made materials derived from sustainable raw materials such as rayon, as well as non-toxic dyes, biodegradable fabric softeners and natural enzymes for finishing."