Shaneco.com is a good site. If they don’t have one in the location you’re at, you don’t pay sales tax.
Find out what his budget is. I don’t mean to be presumptuous, but a 2 carat stone, particularly a cushion, is going to set him back about 14K just for the center stone, and likely a few grand more for the setting, plus warranties, etc. And, depending on his age and what he can afford, that may very well be too high and you don’t want to get your hopes up. Then again, he may be 40 for all I know! I know my husband couldn’t have afforded something like that until he was in his 30’s, and he was 25 when he proposed.
An H stone WILL be slightly off white. You will notice the difference. I have a D cushion and you can really see the variance of it when you hold it up to other diamonds. Then again, that may not bother you. And S1+ inclusions, depending on where they are, may be a great find or not. S1 is one of the lowest inclusion ratings, and if you’ve ever seen a heavily included diamond, they don’t really thrill you much. If they’re smack dab in the center, you’ll lose sparkle on your stone. Have you considered something around 1.5 carats with an F coloring and maybe a VS1 clarity? That’d still be a substantial stone with more fire and brilliance.
Also, cushions have a smaller face than other stones. So, a 1 carat cushion has less face than a 1 carat round. You don’t really notice unless you compare side by side, so if you’re going for size, you’ll get more bang for your buck with a round or princess. I wanted a cushion, too, though, I just think they’re unique and sparkle very differedntly than most other stones. And in reality, nobody notices that it’s 1mm smaller in diameter.
Start looking around on general websites to get an idea for prices: jamesallen.com, shaneco, even ebay just to see. Your Fiance may be able to buy a used center stone off an antique ring or someone who’s getting divorced, too, or upgrading their wife’s ring! The stores will definitely be more expensive, though. If you have your heart set on a large, quality stone, you’ll definitely have to do a LOT of looking around for a good price.