Most typically fold over elastic is used for making undies, both in mass manufacture and in home sewing. The likelihood is that there`s some FOE present in your undies-drawer as we speak. If you`re interested in giving it a try, there are two PDF sewing patterns available for free download on my blog here. One is for making pants/undies/knickers and one is for making a vest/camisole/singlet, both are designed for knit fabric.