I don't think there is a 3x7 that is made by C&C, but you can make your own. It's really easy!
http://www.cavycages.com/cubes.htm http://www.cavycages.com/howto.htm http://www.cavycages.com/where.htm
You can get the cubes at stores like: Target, Meijer, Bed Bath & Beyond, Linen N Things, Wal-Mart and so on.
The coroplast can be found at your local sign shop, just open up the yellow pages and start calling around.
If you have any questions along the way feel free to ask.
EDIT: Doh. I just saw that you were for Singapore. It might be easier for you to find the cubes on e-Bay then.