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.
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.