Generic sliding door replacement tracks are most commonly made out of stainless steel or aluminum. Additionally, tracks for some windows and patio doors are made from vinyl or other plastics.
Replacement tracks for any given sliding door are usually available through the manufacturer. In addition, hardware stores often stock replacement parts for common types of sliding doors. Replacement parts can be difficult to find when dealing with a custom-made door. If the style of the door differs greatly from conventional styles, contact the manufacturer directly, and order a custom replacement track.
The material of a sliding door is often chosen based on environment. For doors that open to the outside, vinyl and stainless steel are most commonly used. Vinyl is very durable and low-maintenance and can be clad with wood on the inner surface. Because of this, vinyl is most often used for patio doors. Steel is another good choice, offering the same durability. However, steel is vulnerable to denting and difficult to repair. It also requires occasional treatment to prevent rusting.
For sliding doors inside the house, such as shower doors, aluminum is the most common material. It is lightweight, durable and does not rust. Like steel, aluminum is vulnerable to denting, but it is much easier to repair and cheaper to replace.