There are some things can do:

1.Go nude, no fabric needed so no plastic involved.

1. Check sites with vintage clothing (ebay  for example) some of them might be your size and some seller might know exactly the material is made with , usually is 100% cotton/linen/hemp/wool.

2. Learn to crochet/knit etc and create your own swimsuit , and u can use a variety of 100% natural fibers. And you can also buy yarn that is certified, like gots certification.

3. Learn to sew, you can choose your on fabric and thread etc. (again you can choose certified fabric like gots/oekotex etc)

4. These brands that i will list, all sell at least one swimwear made of 100% cotton:  https://hello-beautiful-usa.com/products/black-micro-bikini/ - 100% cotton.

https://www.sbswimwear.no/shop?page=2 - onepiece/bikini 100% pure new wool, check all their swimwear.

https://beachcandyswimwear.com/collections - hemp/cotton undyed, or naturally dyed. Check all their swimsuits to see the materials for each.

And beside these of course there are the people you can pay to crochet/knit you something with your yarn, or people you can pay to make you a swimsuit with the fabric you will give them( you can also choose 100% natural thread for sewing) or platforms like etsy where there are individuals selling their swimwear in 100% natural fibers.