Do They Wear Undergarments?
The Amish can be categorized into several groups; the extremely conservative Swartzentruber, the Old Order, the New Order, and the Beachy. For the last three groups, there is no restriction on whatever underwear they want to wear. They can go to the store and buy whatever they need.
However, the Swartzentruber have to make their own. They use comfortable fabric to make innerwear that can be held up using drawstrings. This self-sufficiency goes in line with their belief in keeping things simple.