Helmet Foam Inserts?

[Bidoofdude]

Hi all,
I've started the head of my Thundercracker costume and it's pretty much finished aside from colour.
But, as I have noticed, it wobbles around on my head and is pretty hard.
I would like to use foam that fits, to your head, kind of like a skateboard helmet, but I can't seem to find any.
I'm thinking just surround the insider with thick, soft foam sheets to cushion my head and get rid of any wobbling, which seems like the best bet, since I can cut inserts if I need to make any changes.
I think this would do better in the creative section, but I would like to know what you think the easier/cheaper option is.

[SMHFConvoy]

Have you tried Clark's Rubber? Other than that you might need to check furniture upholsters.
If you're looking to do this cheap you may want to go to places like reversible garbage. It's Sydney based but I'm sure there's something like that in Melbourne.