For those of you who have also spent an hour here and there in search of PIMM's in Singapore, like myself. I can at last say that after 23days of looking I have today found a shop which sells it, albeit not cheap S$63 (but alcohol in singapore is NOT cheap!!!)
Anyway it is in a shop called Bacchus, (wine shop) which is located in Basement 1 of the Paragon Plaza.
Happy shopping & drinking