Here’s a link to purchase the knobs on Amazon-
https://amzn.to/2KcL5hF
Product description
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.