I get mine from ebay.
- looks like these guys have a better selection.
What I don't like about the cake dummies on ebay is that they're only 3" in depth, which isn't very high. May be better trying a specialist company such as the above link if you wanted a greater depth. Although obviously the more expensive option.
The idea I just thought of is to get two the same and stick them together if you were looking for more height.