Any thrifting places are a really good source. I get most of my alt stuff off of Depop, and then modify it to my liking if I feel like it. Hot Topic is also really good for accessories and stuff, though they might be a little more pricey than thrifting or DIY-ing. Also, avoid directly purchasing anything from Dolls Kill or fast fashion brands. These things show up in thrift circles a lot, and while they might be good for thrifting, they don't last very long and the companies are usually pretty shit (especially Dolls Kill).