Short term you would be well placed to use wood in many of its possible forms. It's the prototype modelling material of many model makers. You can get flexible plywood and bendy timber too. Otherwise visit a local radio-control model shop. They have lots of lovely modelling materials, natural and manmade that can be shaped with simple tools.
If you're feeling adventurous, you could buy a lot of modelling clay and get busy. This is at least re-useable. Once you have your form set,you could then use it as a mould for a fibreglass casting or similar.
Wood is cheap and readily available. Once you have an approximate form that you are happy with, you could try and find a college/company/individual who could then manufacture a next stage model with the many 3D modelling printers that are now available. They can "print" your components and form factors into a more polished product.