Jeans: Anywhere, really. When it's time to get a new pair, I go to the nearest store that sells them. The only exception is whenever I'm in the US, I get some from Target, as I find the Denizen line really comfy and I haven't found them in Europe.

T-shirts: Usually gifted to me. Plus I accumulate t-shirts from work that I often wear privately.

Socks: My local grocery store. They carry the Caterpillar brand which I really like.

Underwear: wherever.

Anything else: I rarely wear anything else

Uh, I wrote about this a while ago: https://lemmy.world/post/39198020

Either online or charity second hand shops in my town.

I usually check out sportswear stores to get something with any colour in it. Cannot recommend, they of course charge extra for their stupid logos, but I really have enough blue jeans and black t-shirts they sell everywhere else.

I have been getting all my clothes on vinted or etsy since a couple of years