If you need a flexible material, there are a few options you can choose from. The cheapest would be a TPU processed by Multi Jet Fusion. It is a strong material with a broad range of application. Be it springs, energy absorbing cases or shoe soles.
If you need something that is more heat resistant and with a broad range of different degree of hardness, you might be looking for silicone.
Look at our silicone article or TPU article for more information.