Jim Rubberized Cork material can be found at companies selling and manufacturing custom gaskets and seals. You may not be able to find the total thickness you want but you could build it up and the form to shape.
If you want a solid rubber tip look to Medical supply stores and you may find a replacement crutch tip or cane tip that would fit your requirement. Some metal furniture also have tips of the type you are looking for.
Good Luck saltRon