I have seen many cake dummies being sold but I have not seen any suitable for stacking in between the tiers and placing flowers around them. Are you meant to cut this to shape from the cake dummies? This will be quite tricky. Also, how would you stick roses together to create a round ball shape centre piece on the top tier? I am guessing from a ball shaped styrafoam or a ball of fondant.
Approach the makers of the 'dummies' - the ones we have here in Australia will cut almost any shape you want - at a COST of course!!
Are you talking about the "Separator" type dummies - because they are just a thinner version of your normal shape ones.
We can purchase dummies as thin as 1inch here so its easy to make up whatever size you want simply by combining them
You could use oasis instead. If it's for fresh flowers you can use the wet oasis as the flowers won't wilt but obviously make sure it doesn't come into contact with the cake. They also do a half ball shape of oasis that you could use for fresh or sugar flowers.