this would be a good deal provided u have siblings still schooling in MOE schs(pri/sec/JC only).
Or if u know anyone working in MOE...also can...but if yr works are for commercial purposes, some have cautioned that there will be an inbuilt signature into your works, which can trace whether the copy is a educational(thus non commercial copy, strictly not for commercial usage), or commercial one.
Cost wise, this avenue is the best deal....cannot be beaten.
If you are talking about store fronts, like wad was mentioned, ricebowl, the book shop on the first floor of funan beside subway carries it. more costly though. Buying from poly would not be that much cheaper