Hi Cemily,
We got our son's bed from IKEA with a barrier and you have a choice of matresses (I think). They also sell children's beds at Bumps to Babes in Ap Lei Chau. I don't know if that's any help.
Art Deco in Wanchai and Happy Valley sells custom-made beds with mattress. You can custom made everything from size to color to the artwork on the frame but of course it'll be more costly.