A Toyota Avensis has a key that does double duty as a remote keyless entry fob. With regular use, these keys can become stuck or worn from use. If you'd like to replace the buttons without replacing the entire key, then ordering a new set of buttons or scrapping a used key is the best way to go. Otherwise, you will pay for an entire replacement key, which would cost you considerably to have cut and re-programmed.
Purchase a used key online from a dealer. Since you are using the key for its buttons, it doesn't matter if the transponder works correctly.
Turn over your old and new Avensis keys so the Toyota logo is facing down. Toward the metal key shaft on each, you will see a small Phillips head screw. Loosen these screws.
Remove the button plate from the keys. Place the used key buttons into your working key and close.
When debris gets caught between the contacts and the buttons, the key will no longer function. Take this opportunity to clean your key out with canned air. Change the battery while you're at it. A watch battery will work. Be sure to insert with the positive side up.