James Allen and its comoetitor, Blue Nile Diamonds are both good options.
https://www.bluenile.com/diamonds?click_id=272148353 Both offer quality far beyond anything you can get at a mall store. The thing to remember is that diamonds are fungible. Whatever size and grade diamond is identical to another diamond of equal size and grade, so you need to find the most efficient way to buy the diamond you want without paying for any brand name or get fooled on quality. That's where the online stores shine, so to speak. They price so precisely that you can tell the quality you get almost by one dollar increments. Years ago I bought a nice tennis bracelet for my wife from a web site called Dirt Cheap Diamonds. Don't laugh, they were profiled on NPR. They were outstanding and educated me. Since they went out of business we have bought from Blue Nile with the same results. But since then we discovered the auction sites and the quality and value you get through them is even greater. You just have to wait a while to see what you want. But eventually anything you could want will come through one auction or another.