I don't see the point of risking to buy a bag which is cheaper and worried about its authenticity.
Even if u manage to find one, how much can u save? $200? $300?
Consider all this time and effort u spend to find this "cheaper" deal, and after that when buying, still worry about its authenticity, because nowadays there are so many "Grade A" around.
I rather spend more to buy in a boutique with its receipt and sleep with a peace of mind everynight.
Simple, easy to find, and peace.
And if u really want cheaper, the only way is to buy overseas. Which u already stated NO, u want to buy in Singapore.