I started with budget, then went to the WF and BGD websites and looked for the largest size ideal cut diamonds within my budget and then narrowed it down by the clarities/colors I was okay with. Both sites have an easy way to find matching pairs so it's really a breeze...
I assume it's round diamonds you want? Gypsy again nails it with the easiest way to go about purchasing matching round studs. Alternatively, if you can't find quite what you are looking for, contact a vendor you would like to work with whose opinion you trust, tell them what you are looking for and budget, they will be able to find a nice pair for you that fit your specs from diamonds listed on the main database if there isn't anything in house that fits the bill.
Good luck with this and if you need any extra help or would like us to look over some diamonds for you, just ask!
I also bought my pair from WhiteFlash. It is easy to buy there because their stones are cut consistently and easy to match. What is your budget? We can show you some options.