Clothing - linen, merino, cotton, polyester?

I personally am not a big clothes shopper, I pretty much replace underwear/socks when they break or I lose them, and once per year, I buy some clothes. Yesterday I bought my first linen T-shirt. And it feels so airy and comfy, it's amazing and not itchy. What do you guys prefer to wear?
Where do you guys source your clothes, also in regard to ethics - no child labor. I guess second-hand clothes are the best in that regard?
Does anyone here feel strongly about a specific material? I heard merino is supposed to feel amazing, but the price tag keeps me from getting a shirt haha.