You might try Minox Photo Labs, aka MPL, in New York state -- they do most of the mail-in processing for Minox films in North America, so probably tend to have lots of cassettes around. Another place to try is Goat Hill Photography, who sell cassettes and reloads for the more common submini formats.
No idea if any of these places ship to Singapore -- you might also check if there are local dealers who sell Minox cameras; if so, they're likely to have both loaded films and empty cassettes; even slitters.