If you haven't already, check with Bodek and Rhodes. They are a huge wholesale supplier to the imprintable apparel industry. We used to buy a ton of stuff from them when I was in the touring entertainment industry, and they generally had everything from rock bottom Chinese shirts that were thinner than toilet paper to some pretty upscale golf shirts, dress shirts, satin jackets, and the like. If anyone has nice boxers in bulk, it would probably be them. Otherwise, you may have to buy in retail packaging.

Staton Wholesale is another one we occasionally used, and there's another big supplier we used to deal with that has warehouses all over the US, but I can't for the life of me remember their name. If I can find my old paper Rolodex, it's in there and I'll post it.