I have been looking for something along these lines for years now. You would think that a bikini in the shape of a pentagram would be standard issue for every online goth retailer (and I frequent a LOT of online goth retailers), I've only seen one before and it was sold out before I could buy it.
Like all the best labels, "goth girl" is one that is best when self-applied, and I don't know if it really does apply to me? I check all the superficial boxes I guess. My favorite colors for clothes are black and white, my hair is dyed black, my preferred makeup look is just black, I listen to goth metal music (in phases, not always), so fine, I could at least be mistaken for a goth girl if I were out in public. I was inadvertently the subject of a local news story because I was shopping in the grocery store and some people thought I was a ghost haunting it. That's a true story.
Anyway, I've really wanted one of these for years and they are just impossible to find for some reason. There's a kind of ecosystem to the goth clothing market that I find really interesting. It's like a forest. You've got these big trees making up the bulk of it, that's your major retailers that design original clothing and sell it with their own branding. In the US, that's Killstar, but there are other, bigger ones, mostly in Europe. They make big purchases of stuff and hold it in inventory for a long time, and, crucially, sell it to smaller stores.
Then you've got the smaller plants and aspiring saplings. They do small buys of really niche, unique stuff. They sell out fast and usually don't make second orders but sometimes they will if something goes viral. I really love Rogue and Wolf's stuff, and you generally can't get it anywhere else.
Lastly, you have the mushrooms and fungi, which is not meant to denigrate them at all. They don't produce their own stuff but they buy up old stuff and centrally locate it so it's easier to pick over. That's where I got this swimsuit. They buy a decent selection of stuff from all over the ecosystem, raise the prices a bit, and keep stuff in stock for longer than the other two types of stores above. They normally have TONS of stock, but only a handful of pieces of each, that they usually get at a clearance discounted price. If one of the "big box" stores has something you like, but only in S and XL, you might be able to snag a M from one of these places if you move fast.
I don't buy as many clothes as I used to, mostly because money, and also because crop tops are in fashion right now and I don't like them, but I stay subscribed to all these newsletters because you never know when you'll hit on something awesome.
I remain genuinely surprised that no goth stores have even remotely attempted to capitalize on the fact that a very popular(?) cartoon character wears basically this exact thing and sell clothes of it. Instead you just have to get lucky on a swimsuit design.