The transparent TPE flexible filament from 123-3d.co.uk has the right flexibility for most FDM printers. It has a shore hardness of 45D. TPE filament is characterised by high elasticity and excellent wear resistance. However, the print speed is limited and not every extruder is suitable for this material. So, first check whether your 3D printer is suitable.
Read more about printing with TPE filament
here.