Best Time For a 3D/4D Ultrasound

Liz
I'm currently 22 weeks 5 days and I want to at some point have one of those really cool 3D ultrasounds done. But we can really only afford to do one so I'm wondering when is the best time to do it? Like, not too close to the due date but far enough along for him to be pretty much developed to the point of how he'll look when he's born. I'm due Sept 21.