I'd look into diy-ing (if you have sowing needles, thread, etc. at hand and/or are willing to buy fabric dye) that's my way to go! It's fun to transform the stuff you already have laying around at home.
Other than that, I think Killstar, Eva Lady (more romantic goth), Punkrave and EMP (they sell band merch mostly, however they also carry lots of alternative fashion! If you want to look into that too) are pretty solid!
Edit: as the poster before more already mentioned, do stay clear off of Shein/Romwe/etc and Dollskill. They steal from artists and honestly the quality is not worth it. Dollskill might be better in quality, but it'd leave a bad taste in my mouth to support a company that prints shirts saying "Goth is white".
Aside from that,
Good luck! :)