I agree (and believe) that companies have the capacity to change for the better, and consumer pressure through social media or activist campaigns is most of what drives them to change. For example H&M's record on waste and worker safety/wages wasn't spotless in the past, that was publicized, and they made some changes - I find that really heartening.
I feel conflicted about it to, because with the price points I'm working with, I'm often attracted to the fast fashion retailers for the most contemporary shapes. I can try to selectively choose items and care for fast fashion so it lasts a long time and gets heavy wear, but that still doesn't address the way it was created (or what the money supports based on the lobbying contributions of the founders as Laura points out). I really appreciate these threads because they can make me think twice before buying and help me alter habits.
Some places to buy that are pretty good guys or support local workers/makers -
-A lot of the outdoor gear companies, as Laura points out - Patagonia, Timberland, I think the North Face?
-Etsy (if it really is an independent artisan, they have started to blur the line) and local craftspeople at craft/art fairs. Good for jewelry, scarves, bags in particular.
-Local small designers, and bag makers, and graphic t-shirt printers (If you live in a bigger city, someone is making messenger bags and bike bags locally and screen-printing tees).
Buying second hand - no additional use of resources in manufacturing and little in transportation with brick and mortar stores. Keeping clothes out of landfills:
-Goodwill (employment and job training for people with disabilities and veterans), other charity shops. There are some watchdog criticisms of Goodwill, but some of them are based on inaccurate info, and some are contested by different advocacy groups -- It seems to be complicated, but I agree smaller local charity shops probably are a good alternative.
-Buying second-hand from locally owned (usually women owned) consignment shops and buy/trade/resell shops targeted at young people.
In general I look up a company on wikipedia if I'm curious about their record, it's quick and revealing.