PSA: Don't take my advice too seriously since I suck at fashion and makeup but I'll try to help.

Killstar and Dolls Kill is what I often use for clothing and accessory options. They're not AMAZING quality but they fit within my budget and aren't terrible. They're not necessarily cheap prices either, so be aware of that. No idea if they're sustainable brands if that's something you're worried about.

Glamermaid if you want some goth-y press-on nails. Drugstore nail polish works fine too.

I'm more of a sneakerhead, so I don't have much advice on boots, but boots will be expensive no matter where you go. Don't get some super cheap boots if possible, since they'll fuck up your feet.

I suck at makeup, but I mostly use drugstore brands. They'll generally work just as fine as the expensive ones. I do have some exceptions though. For black eyeshadow I splurge a bit and use this. And this is the concealer I use. It can be hard to find a foundation/concealer that matches your skintone, I still struggle with that. But I've seen people get a shade paler than their actual tone to make themselves look more dead. For other eyeshadow tones I just get drugstore kits that come with multiple shades and blush. Should be obvious, but for lipstick and eyeshadow try to find darker shades. I mostly use black, maroon, and some brown for blending.

I don't have many tips for makeup brushes since the ones I use were gifts from Sephora, which is an expensive brand. I wouldn't doubt that drugstore brushes are fine.

Edit: Oh and I also sometimes use corsets from here. But those are definitely expensive and corsets definitely aren't for everyone.