Have you tried sourcing some in Europe ?
We sourced some in the UK a little while back - posted in another thread.
They got many options of construction material and sizes, got about 12 for £20 (gets cheaper the more you get)
We were just seeing how useful they would be before committing to any more, can see using them to bulk out mountain builds but we have yet to make a decision if we need more.
Was reasonably impressed as they did the job I was after - link to flickr album
If we wanted more we would try to assess how many we need, what size (we chose 4"x4"x2" - the size of a CBS building block) then compare price against dice4hire's options (we think dice is cheaper).
The sight we used was https://www.efoam.co.uk/
and the foam type was 'high(firm)'.