Articles for tracking generally include wood, leather, plastic, carpet. Schutzhund regulations specify specific sizes for trialling. These are 10cm (4 inches) long by 2-3cm (¾ inches to 1¼ inches) wide by 0.5-1cm (1/5 inches to 2/5 inches) thick. You can buy articles specifically made for the sport at various retailers including Gappay, but you can also make your own from offcuts and samples.
When training young dogs, a material which holds scent well, such as leather, makes a good starting article, preferably around double the regulation size. If you begin by training the articles off the track with a clicker, use the opportunity to introduce all sizes and materials but be sure to scent them well.