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/

You can contribute funding to Greenpeace legal defence here:

https://donate.greenpeace.org.uk/stop-shell-appeal-n-too/~my-donation?ns_ira_cr_arg=IyMjIyMjIyMjIyMjIyMjIyMjIyMjIyMjIyMjIyMjIyPw7Ect235ai5hAk0GW7J2f43V%2FrqosAN5vHS0uhTlqLvxwVSl%2FWn9pcidMCBOUpb2DL9x36zx5m44X2%2BgwR4mC&_cv=1

It'd be great if posts like this could include a) the location, b) when it was taken and c) who the photographer was.