Richardson: Bruce Miller Nursery
Dallas: Northhaven Gardens
Redenta's (sustainably raised plants, too)!
Soil Building Systems - Best compost in town. You can get great products in bulk, bagged, or delivered. They have a variety of rock, mulch, soil selections, and amendments (including "Gumbo Buster"). http://www.soilbuildingsystems.com
Carrollton: Strong's Nursery & Garden Center
Frisco: Shades of Green (sustainably raised plants, too)!
Pardner's Garden & Feed

This is not a complete list by any means, but I wanted to mention these sources.

Happy Gardening!