Jockey
Specs partially disclosed jockey.com ↗
Jockey is a conventional underwear maker, not a plastic-free brand, but the Elance 100% cotton line is a genuinely useful budget pick: the fabric is 100% combed ring-spun cotton with a 100% cotton gusset, and the waistband and leg openings are covered elastic, so only cotton sits against skin. It isn't zero- plastic - the covered elastic core is synthetic - and most of Jockey's other lines are synthetic blends, so the Elance cotton styles are the exception worth seeking out, valued for being cheap and stocked everywhere.
How clear are their specs?
States "100% cotton" fabric and gusset and describes covered elastic waistband/leg openings, but does not disclose the elastic core's fiber.