It could be foam insulation, and yes, you might find it in a hardware store, especially a larger one.  Here in the US, we have two hardware store chains, more formally referred to as "home centers", or "building centers".  They sell pretty much anyone needs to do most kinds of work on a house, both in terms of tools and materials.

I don't think I mentioned this before, in this thread, but another source is construction sites.  I don't mean scavenging at a building site, but if there are any building sites near you, walk past and see if they have any scraps of foam insulation.  If they do, find the foreman or supervisor, tell him you're a modeler and you use foam for groundwork, and ask him if you may have some.  I have found consistently that they're always happy to let me take some.