Hi Nan,
like that Fat ger said, what is your budget?
It really depends on your budget and what you are looking for.
Are you looking more for size, colour or clarity? or the brand?
Most times, maybe you can go for the size & colour. For the clarity, you do not need VVS1/2. Even VS 1 & 2 will do just fine.
Shops like Lee Hwa etc have their own inhouse designs (not sure whether am i putting it the across properly) like Desti*** or someother name (cant remember now).
Whereas Tiffany has their own inhouse certification for their diamonds, and they do not use GIA cert (like most of the other jewellery shops).
hope i explain myself clearly.