You should be able to get filler at any Do It Yourself store (home depot, lowe's etc). You will probably want Ready-mix filler, not the kind you have to mix yourself.
It's a construction material used to fill gaps, even out two substrates, etc. It's pliable which is why modelers like to use it.
Just head to one of the above stores and ask where the ready-mix filler is - they should be able to help you out.