Modern underwear peeking out from guys' pants is lame. Just buy a pair of these correct (and inexpensive) cotton muslin drawers based off an original set worn by an Army of Tennessee soldier from Alabama, and reproduced by South Union Mills. A basic citizen's style of drawers, these would work well for any impression, not just AoT use.
AoT Drawers - We Soon Got Proud
$60.00Price