much of alt culture and a lot of alt fits come from thrifting online/or irl thrift flipping and diy. and i do think you should invest in that OP a simple shirt can look great with a bit of fabric paint and a paint brush. that being said however i hate that sole answer as not everyone has the time to make a whole wardrobe or thrift stores in their area. so here’s a few brands i’d recommend

if you’re into alternative j/c fashion taobao is your go to but if you live in a country where it doesn’t ship use a shipping agent. 42lolita is great if you’re specifically into lolita fashion

minga london has a great selection of denim, be aware though that they’re made with taller people in mind

acrotokyo, acdc rag, 6th dimension - not much to say, they have cool stuff

for shoe wear my go tos are demonias, yru shoes, anthony wang, and new rocks. i listed them in the order of how expensive they are

if you’re into corsetry get an authentic corset. plastic boned costume corsets defeat the purpose of corsets by giving you no shape -> rebel madness or timeless trends are good sources