I don't know whether Singapore allows importation or not. It probably does, but even so I doubt you will find anyone willing to ship to you. Most breeders, if they will do it at all, will only ship internationally if they're selling an order of large monetary value--otherwise, it's not worth all the hassle.
I think we had a member here a while back, though, that was living in Singapore and that person was able to find quite a few cornsnakes locally if I recall correctly. Not as many as we can get here, obviously, but a few different morphs.