Hi Jocelyn
I got mine from Bangawan, the chocoloate flavour one. Personally I feel that the cake itself is not "nice" looking and their service is not that good too. If I were to choose again, I'd rather go for the traditional cake combination you mentioned.
I think you can go to those normal HDB confectionary to check out if they have, I believe they do. I heard from a friend that not too sure whether is it at Bedok or Hougang there is one such confectionary who sells that, and the box only cost about $4+. They will also sell items needed for Guo Da Li, like the Long Feng Zhu, biscuits, that type lor... so I assume if you go to those chinese looking confectionary (Bing Dian) or perhaps Chinatown, you can still find those things.