Bing found the following results
Amazon.com: Replacement Shower Knobs
Amazon.com: Shower Knobs Replacement Kit 3 Piece
Amazon.com: Knobs For Shower
Shower Faucet Handles at Lowes.com
How To Replace Shower Knobs - Ace Hardware - YouTube
Delta Clear Knob Replacement Handle with Chrome Arrow Button for Tub ...
How to Replace Shower Knobs (with Pictures) - wikiHow
Tub & Shower - Faucet Handles - Faucet Parts - The Home Depot
Tub - Faucet Handles - Faucet Parts - The Home Depot
Replace/Upgrade your Shower and Bath Handle - YouTube