Honestly, best place to start is a rookie Loner or Bandit with some more muted tone jeans, and a well worn jacket that matches the vibes and tone of other stalkers, like khaki or a drab green (black leather jackets for bandits is a classic too)
Thrift stores are the absolute best places for these kinds of things. They're already pre-worn in so that they have authentic weathering, and they're way cheaper than buying brand new clothing. It's pretty easy to find the right stuff, just need a bit of luck and patience!
As for gear, older military surplus stuff is pretty easy to come buy if you look in the right places, just do some research online or look for nearby surplus stores and whatnot, and choose something that would fit your character/loadout. Just don't be that guy that buys stuff that doesn't fit the STALKER vibes, like a cheap JPC knockoff or something lol