To be frankly tell you, getting those diamonds from "KNOWN" brand confirmed will be expensive. They tend to mark up the price due to the "branding". You can try to find those shop who is generally customize the ring, and definitely you can get whatever shape of the ring that you want as long you provide sample to them.

Beginning of the year I bought from a unknown brand in Singapore called JannPaul ( not Love & Co, Tiffany & Co, Goldheart etc ). I bought 0.59c diamond with Class E and VS2 only cost SGD5.9K. Nowadays GIA is quite common for all the diamond, and those vendors usually would avoid those without GIA cause they know that its very hard to sell them off.