From what I've read there is a famous place in Singapore called Mustafas - this is apparently a place that sells loads of stuff but its also known as THE place in SIN to get shaving gear. I dunno if you'll have time to get to see it but I've read repeated reports that they have a fair bit of stuff and it's very, very cheap.
This is exactly the kind of thing I was looking for, thanks mate. I think we're going to be in Singapore for about six hours on the way back from Cambodia, and this place looks like it's a convenient 20 minute drive from the airport. I'll try to stop in there and find some cheapies.