As a wife who is in charge of purchasing said underwear it depends on what you're looking for. My husband favours loose boxers and we are both disinterested in brand names more about comfort, so he normally ends up with plain black ones from Matalan, Tesco, Asda, Primark, with a high cotton content and loose leg.

Most supermarkets do their own range of underwear, Marks and Spencers have pretty much every sort under the sun. Sports wear shops will probably have branded, but obviously more expensive. Pretty much all menswear online retailers will have them. Just make sure you check the sizing guide because a medium from M&S and a medium from Shein are likely to be pretty different.