Heel counter
Internal or external, the heel counter is the pattern part that covers the heel of the shoe. The internal heel counter can be made of rubber (for vulcanized shoes), thermoplastic (for cold cement shoes), chemi sheet (for really cheap shoes), or leather for dress shoes. Depending on the shoe type, the counter can be thin and soft or stiff and sturdy.

You can buy this material here: DIY Shoemaking Parts Super Kit