ASTM Method D-2047
“Standard Test Method for Static Coefficient of Friction of Polish-Coated Flooring Surfaces as Measured by the James Machine” lists the following items as being sourced by the Consumer Specialty Products Association (CSPA) or Chemical Specialty Manufacturers Association (CSMA) but are no longer available.
Tight-Tolerance 6061 Aluminum with Certification Precision Ground on Top and Bottom, 12″ x 12″, 3/8″ Thick, Part Number: 10100.67353
Note the method advises 0.125” but the aluminum can warp and have a negative impact on testing. A thickness of 0.375” may produce better results.