http://metromaples.com
Pros: They have Shantung, Trident, Ginnala, Kyohime, Deshojo, rough bark Japanese maples, and others. Reasonable prices.
Cons: Their maples are
not prebonsai and most are grafted; expect to air layer and invest time in developing roots and a trunk. I think about 5 years for a $15-$45 maple to start looking like a good prebonsai Japanese maple that might sell for $150 or more.
Also there's Bent Tree nursery in Dallas. Kinda of bad looking part of town. Don't let the look of the place discourage you. There's some good bonsai stock there. I'm impressed at the large Trident maples they have. While there, check out their twisted pomegranate.