if you are into the authentic goth stuff and also agree with this subculture's values, you should really look into upcycling clothes you already own or thrifting
there are a lot of tutorials online on how to make gothic stuff out of non-gothic clothes, and there are some amazing pieces you can find in your local thrift shops and on apps like grailed, vinted, depop etc
if you choose to buy new, i would suggest supporting small businesses. idk which specific style of goth or alt you are looking for, but you can always find good stuff on etsy (it can be pricy and recently there are a lot of dropshippers so be careful) and here are some brands i know personally: new rock, disturbia, church of sanctus, tripp nyc, punkrave, banned apparel
but always do you research and, if possible, try not to support brands that are hurtful in any way