Under Armour used two Cowboy players in ads for their products: Eric Ogbogu and Julius Jones.
Their products are available at several Sporting goods stores: Dick's Sporting Goods, The Sports Authority, Hibbett Sports, Modell's, and The Sports Chalet.
Check the sites below for locations near you, or for online sales:
I did a search while I was on SportChalet.com and I found nothing with Cowboy and Underarmour together. I did see Terrell Owens' Jersy replica!