Zero Waste

Thrift stores are probably the most ethical because it's all donated and second hand. It's already been created, bought, and worn before you see it.

The selection isn't bad at all. Just go look.

I've always thought second hand/thrift stores were great for picking up stuff cheap. The trick (at least in my area) is to hit up the ones in the nicer areas of town, the selection is usually better and it hasn't been picked through as much.

Edit: Just to add, the day of the week, and even season matters. Some places tend to put out new stuff at certain times, some just toss stuff on the shelves as they get it. So sometimes you can go one day and make out like a bandit, other days there's squat. You just have to visit a place a few times before you get a feel how they do things. Also mornings are usually better than afternoons.

Also garage sales. Same basic idea applies, the garage sales in the nicer areas usually have nicer stuff. The earlier you can hit them up the better selection you'll find, but you can often get better deals near the end of the day when they just want to get rid of something. (I bought 2 decent dressers, 2 shelving units, and a small desk for $40 last summer just because the guy didn't want to have to drag it all back into his house.)

Thrifting is always best IMO. The selection depends on your location; I've noticed smaller, more rural areas tend to have better selections than suburban areas or cities. There's also online thrifting through sites like thredup and vinted - obviously, your mileage may vary. I use FB marketplace to get home goods but I'm sure it's good for getting local used clothes, too.

If you want good results while thrifting it pays to get comfortable with going regularly. If you go consistently and pick through stuff you'll find great stuff, but ultimately you're picking through a haystack for the couple of needles you care about- youre getting cheap clothes but putting in your time. Also its way more fun if you go with friends, and then you can also learn what each other like and keep an eye out for each other :)

I've always thought of Patagonia being decent, does anyone know of ethical concerns with them? They're pricy though, and only have specific kinds of styles

If you're in America..

https://www.market.unicefusa.org/

As others have said charity shops are definitely a good choice. Here's two websites that list ethical/environmentally councious clothing companies if you do want to shop brand new:

https://www.ethicalconsumer.org/fashion-clothing/a-to-z-ethical-fashion-brands

https://thegoodshoppingguide.com/subject/ethical-fashion-retailers/