Kallistra sell a pack of three (non-hexed) hills that could double for dunes, though they are large, depends what scale you want. LINK –
Also, Rymans (UK) sell that stuff that looks like a bandage that has been coated with plaster. You briefly soak a length of it and then apply it to whatever form / shale you have created and it dries hard. It does have an open weave, so when dry, rub some pollyfilla type stuff into it. It could then be PVA’d and dry fine sand applied.