From what I can see, these "cales" to support the "platelage" tend to be made of plastic or rubber.
See:
http://www.terrasse-nature.com/PBCPPlayer.asp?ID=297557
(there's quite a good picture where you can see some black round discs. I think these are the "cales")

Here is another picture that shows quite clearly what these "cales" are:
http://www.terrasse-nature.com/cales-pour-terrasses-en-bois....


Now that you know what they look like, look here:

http://www.ecobuild.co.uk/var/uploads/exhibitor/149/la8pdpth...

It says that these are called "support pads" and there seem to be different shapes available but the main feature of these appears to be to suspend the paving or timber to allow drainage, and I could swear that I read somewhere that that was what "cales" were used for.

If you do a search on "decking support pads", you'll see plenty more examples.

I am not 100% sure about this but it seems that this could be the right term.

https://www.google.com/search?q=joist blocking&rlz=1C1CHVV_e...

français term (edited): cale

In the absence of more detail about their shape and/or size, I'd say this would be the safest solution; note that 'packing' itself is uncountable, so if your situation specifically requires a countable noun, you'll need to use 'packing pieces'.