Here’s a link to purchase the knobs on Amazon-
Strength, durability, and heat resistance are some of the benefits of this broad range of phenolic handle. Extremely versatile range of mountings. Female insert machined to suit. Mounting type: female insert. Thread length/depth: 3/8″. Mounting diameter and threads per inch: 1/4-20. Plastic material: thermoset; insert material: brass. Measures 7/8″ diameter by 2-41/64″ height.