I would assume it would be different then an engagement ring type of diamond. Hi Glenn925, Most of the diamond vendors will be glad to provide you with the necklace setting.
Some jewelers do also a custom design.
As for the second part of your question, let's wait for the expert opinion. But if you feel okay with dealing on the web, and you don't mind emails and phone calls, then you can save a couple bucks, make friends, and get what you want. I hear this all the time, and you may give a little, but keep them nice and well cut. Cut alone will produce the most brilliance and despersion, then I would consider color, then clarity, and last the size. If you feel as you must get a certain size, then slide the qualities where they best suit you.