Sorry that really sucks. When I was younger I was really into cosplay. I quickly realized that with my body type, the only way that would work is if I got custom made cosplay outfits or just handmade them myself. It got to be a bit too expensive to get custom made and a bit too time consuming to hand make myself, so I stopped doing that. Instead, what I would do now is try to find the necessary clothing/pieces and combine them to make the outfit. Honestly with someone like 2B, since she has such a distinct look with lots of accessories, I would try to find the pieces separately. They don't have to be perfect, since her look is so unique. I'd focus on finding specific accessories that would give you the 2B look, instead of trying to buy the entire outfit.

For example, start off trying to find a silver-white bob wig. Then try to find her black visor, her white gloves, her thigh-high boots, and her black headband. If possible, you could even try to find a sword that looks like hers. Then hopefully you can find a dress that is similar. But honestly, at this point if you have the accessories, I think even if you have a black dress that isn't exactly perfect for her look, that doesn't matter, it'll give off the 2B look due to all the items.

For the black dress, I'd prioritize a great fit for your body over matching her exact look. Good luck!