This will likely depend on whether you are a man or a woman, what kind of clothes you want or are expected to wear, your size, your stance on ethics/fair-trade, etc.
As a female graduate student in engineering, in my 20s, I dress quite casually most days. I sometimes need a few days of business-slightly-formal wear for conferences, etc. Slacks and blouse/sweater are fine in my field.
For dress pants, jeans, yoga/exercise pants, some tops I go to discount stores like Ross, TJ Maxx, Marshalls, Burlington Coat Factory. I try to spend $25 or less on pants, $15 on yoga pants, $10 or less on shirts. I've found some great $10-15 pants before. In my experience, Burlington (except the awful one by me now) has a good selection professional clothing, including suits, especially during an annual sale (I think in springtime? I hit it on accident once.) They may not hold up to true white-collar snuff though. I sometimes go to Nordstrom Rack but find it more expensive than the others.
I love t-shirts and cardigans from Target, though I usually shop the clearance and sales. Last week (through Saturday) they had 40% off Merona and 50% off certain Mossimo shirts. I got 5 for $5-6 average (it was time). I've heard the opposite from others, but I find that Target shirts hold up well for several years (and even then, the issue becomes more about discoloration...). Also get underwear and socks there.
Outerwear and certain performance items I go to REI (also trying to hit sales). Recently: hiking pants, sports bra, wool socks, splurge on Chacos. Less recently: winter coat, rain shell, fleece. Ages ago: hiking boots. Can be good to check for certain items on Sierra Trading Post before buying; it's basically the discount REI.
I used to go to JC Penney sometimes, but lately I avoid it because I hate the giant mall in my area. But I've found pants, shirts, sweaters that I like at a similar price point to the discount retailers there.
I fix minor issues with my clothes to make them last; and also save new ones for "public facing" activities and wear older/torn/unfashionable ones around the house or lab to wear them out. My views on the effect of boycotting major retailers/fast-fashion are mixed and complicated; but I figure if I minimize consumption overall, I'm minimizing my effect on the negative aspects.