I hear that 27 weeks is optimal but it sounds like from other moms that you are still in the time frame. My doctors office does them so I will do mine their which I'm thrilled about but....my stenogropher (is that the right word?) is amazing and actually teaches the ultrasound class at El Centro college in Dallas. They do the 3D ultrasounds their as part of the class for about $30-$35 dollars. A friend of mine had hers done there. I called during my last pregnancy (I had a different OB who did not offer the 3D ultrasounds) but the class was only offered in the opposite semester which is now. Not sure if you get dvd's and extras like you would in one of the specialty places (you may only get still photos) but I'd call them and check before spending hundreds of dollars. Good luck.
After posting I looked at http://www.ultraviewimaging.com that Renee listed and they show examples from 25-37 weeks and the later ones actually look better since the baby seems more developed and filled out.