The trade uses a variety of sites for "wholesale" prices. Rapnet is the standard, and they publish the Rapaport Price List. However, it only covers natural diamonds, and rapnet only lists natural. Diamonds generally go for a % discount off the Rap Price List.
Polygon (.net) is the other major listing platform. They include lab diamonds, and most everyone uses them. There are other platforms, but those are the two majors IMHO.
You need to be a member of the trade to sign up AFAIK.
However, I have seen one retail site, Ritani, actually post "wholesale" prices and they seemed to be pretty accurate. They seem to have good prices too. They list their wholesale/markup/etc. Of course, you should buy from a nice local jewelry store, but if you want to buy lab online they are at least great for checking prices.
This looks too good to be true. I am interested in this (because of all the terrible stuff related to diamond mines), but how do you protect yourself from scams on this site? $500-$1,000 is a lot to "test the waters" and see if they are real or even if they will arrive. What is the chance the just swap it out with Moissanite or who knows what?