I never think to buy any but I wear a lot of my relatives’ castoffs. I have a set of long johns from my grandpa and one from my dad, I love them.
So I’m wearing my male relatives’ used (long)underwear - gear is gear, the cheaper the better.
I got some of my favourite T-shirts and sweaters from thrift stores. I don’t bother for nondescript stuff like boxers or socks, though.