Nash's is a sporting goods store on 14th just north of the Tivoli with a good selection of sportswear. They have lots of shoes, athletic gear, and sports team clothes like caps, shirts, and jerseys, including the new DC United jerseys, one of which I bought when I was there.
They also have some sporting goods, like balls and socks and miscellaneous stuff - Nerf hoops, stickers, and so on. The staff is friendly and the prices are fair. I recommend the place, and if you haven't been, check it out. Their website has coupons too.
Nash's Sports is located at 3339 14th St NW, just north of Park Road and the Tivoli.