So once I went to SG (Typically Orchard area), I will try my luck at several apple reseller @ Paragon, Takashimaya, ION Orchard and Wheelock place.
Last Sunday, I went to Paragon's iStudio and seek for iPad 16GB wifi at around 4PM, and the friendly staff told me that I could had brought the last unit if I was there 15 minutes earlier.