Won't help you now OP, but might help other people - Amazon. Yes, I know, but needs must and the devil drives and all that.
I decided I needed new swimwear because I was too fat for my old ones about 3 days before we went on holiday a couple of weeks ago. I couldn't face trawling the shops to pick through the sale dregs, so I went on Amazon and ordered a range of bits in different sizes and brands, figuring that at least one of the 6 I'd ordered might fit, which one (just one!) did. I also found another that would have fit in a different size, so might go back for that one as it was really nice.
Tips/necessary disclaimers.
Read the reviews on sizing. The random Chinese? brands are all over the place. I found 2XLs that were too small and seemingly similar ones where a size L was too big. But the quality was fine, certainly at least as good as supermarket stuff. But they were cheap enough (about £20-25 for a tankini, which seems cheap to me when I'm used to paying ££££s for Fantasie bikinis but refuse to while I'm 20 pounds heavier than I'd like to be but am not doing anything constructive about it).
If you don't have Prime, they'll likely offer you it for free or very cheap for somewhere between a week and a month.
Yes I know not everyone can afford to order armfuls of bikinis to try on, but this is why people have credit cards. You order a few and return what you don't want, which will get refunded before you have to pay the bill. Plus there's also Klarna etc if you must, but I don't know if Amazon have Klarna.
Alternatively, try Decathlon or Sports Direct which should have affordable swimwear year round.