Avoiding synthetics in clothing is indeed difficult. I buy 100% natural fibers where I can, but sadly often the best one can do is ~90% natural ~10% synthetic.
100% cotton would seem to be widely available? Certainly for shirts, t-shirts, jumpers, trousers, socks. I don't wear anything other than cotton really for those items.
For some reason they have started putting stretchy stuff into cotton jeans - maybe style, or maybe (my pet theory) that good quality cotton is no longer economic for jean production, and so they have to use rougher cotton, which needs the stretchy stuff to be comfortable enough to wear.
I think the jeans thing is just because most people want their jeans to feel and stretch like yoga pants and not jeans. The selvedge denim cult still is usually 100% cotton, and about the same price as stretchy designer jeans, but most people don’t want this.