I have commissioned very specific stones (size, colour, type) from the following 2 cutters and have been more than satisfied:
Jeff White at www.whitesgems.com
Richard Homer (through Wink Jones at www.winkjones.com)
For Jeff, you can see my stone in the Colour Me Pink thread.
For Richard, he does a lot of concave work but also traditional faceting, which he's currently doing a job for me. He is very well respected here and has done a lot of work for PSers.