hii! it honestly depends on:
a) where you're based
b) online or in person?
im based in the uk and i prefer online shopping, here's where i shop
online:
depop
ebay
amazon (rarely)
etsy
vinted
in person:
blue banana
charity/thrift stores
brands i recommend:
killstar
hell bunny (quite a bit of gothabilly stuff aswell)
banned
lamoda (shoes and accessories)
hope that helps!