You can't find anybody selling it from Singapore. For sample libraries, the market is too small. I know one place where they used to sell samples (only a few) - but later stopped altogether. They told me that they can't compete with the online stalls which are selling them at already a very low price. Samples is still a very small niche. Within Singapore, it's even smaller. I still recall that shop selling the samples at a higher price than the online stall.
I was in HK in a shop that sold samples - same thing. The price was much higher than if I get it online. And they only brought in a few libraries.
So - online samples are always cheaper. GPO is the lowest price possible. For other samples, always wait for a sale. You get one sale per season of the year (Spring, Summer, Fall and Winter), and occasional special sales for different sellers (4th of July, Halloween, NAMM sales, anniversaries, Christmas, Easter etc). You need to check out all the online shops - some are cheaper than others. Then there's always group buys. Never buy them at the normal price unless you need the sample urgently.