Configure MacBook Air at Apple Premium Resellers in Singapore
"Difference between Apple Premium Resellers and Apple Authorised Reseller": The premium resellers are supposed to have better service and more knowledgable staff.

Unless the Premium resellers stock the CTO (configure to order) models then you will need to order it online.  

The majority of Apple Retail Stores will not carry that model but some Resellers do.  So I would check with the particular store.

As the other commenter stated you cannot physically put more ram into a MacBook Air once it is already put together.  The ram is soldered onto the Logic Board.