Someone mentioned eBay, but I'm pretty sure that several years back they stopped allowing the sale of used undies, they said, for health code reasons. But before they did, I remember some pretty nasty items offered there. For a while afterwards, there were people selling "clean and laundred, per eBay rules" underwear, and then adding at the bottom that the seller could be contacted to "customize" the order, as a way around the rules.
I had a guy, that I was video chatting with, ask me if I took "requests". I asked what kind of requests, and he told me that he was willing to pay for a pair of my underwear, if they'd been worn for a day, I'd cum all over them, put them in a zip-lock bag, and sent them to him.