LTW - The following restaurants serve Wagyu in Dallas -
Nobu, Little Katana Tei Tei Robato Bar, Teppo Yakitori & Sushi Bar
Who's Who Burgers- Upscale burgers (obviously) in a very trendy, expensive and exclusive shopping center.
Chamberlains' Steakhouse, The Mansion on Turtle Creek (used to serve it but they have changed menu's and chefs), The Palm and the Mandaley at the the Omni Las Colinas.
I am sure there are others but these are the ones I readily found and would recommend for your dining.