It has been over 10 years since I lived in Dallas, but two of my favorites back then were Rohde's Nursery in Garland and North Haven Gardens in Dallas.

Dirtdoctor.com by Howard Garrett is a good place to start. Ask alot of questions. The only stupid question is the one you don't ask.

I second Northaven Gardens, they're on Northaven Road a little bit west of 75, the street's just south of Forest. It's a bit tricky to find, it's just next door to the Jewish
Community Center of Dallas, but they're both in the middle of a neighborhood. They're very helpful and knowledgeable.

Calloway's is also a good choice and they have more locations.

Now is the best time to get the trees and shrubs planted, it gives them a head start on growing roots before the summer heat gets here.