Hi Gigabit, you can go to air-con parts supplier shop, I'm 99% sure they sell this (tube or sheet type also have).
I bought mine from this kind of shop.
Just now tried to remove the rear speaker panel (or rear dashboard?), but was not successful.
» Click to show Spoiler - click again to hide... «
I removed those 3 pin clips (blue circles), not able to remove the section where the C pillar or seat "shoulder" was covering it.
Unable to remove C pillar cover as well because the screw cover (green circle) have no screw inside, it is just a hole!

stargate8, these are where the loosing part that need dampening.
» Click to show Spoiler - click again to hide... «
The blue circle area is where rattle when you shake it and also when you turn your air-con to high air-flow.
Green arrow area is also rattling, use whatever way you can think of to make it stay solid and tight.
There 1 more white plastic clip and few more built-in clips at the other side.

To open it, lower your steering wheel, remove two screws on top of the meter panel.
Then use a flat tip screwdriver wrapped with a cloth to pry open from here.

Then 1, 2, 3 pull!
My experimental mod:
» Click to show Spoiler - click again to hide... «
My friend buy for me these LED lights.

Only RM3 per section, quite cheap I think.
I don't know where he bought this, definitely not bought from car accessories shop, maybe shops that sells electrical part?
Below here is my favourites, I stole my mother's clothes hanger to do this!
This is useful *for me* when buy groceries and don't want it to fall here and there etc...

hand of god....