Underwear department of every major sell lines of solid black boxers. They also carry them at outlet mall. They come in cotton, stretch, brief and with legs. Jockey and Calvin Klein have their own line.
Any department store usually sells them jc pennys, Sears, Gap, Hot topic, Dickies, American Eagle, Aeropostale, Kohls, Meijer, Walmart, Hollister, Kmart and many other stores you can buy solid black boxers