You can buy them HERE
Steam Deck shown in pics but they are designed to fit both the Ally and Steam Deck.
Specification:
Compatible with: Steam Deck & ROGALLY
Package Includes:
1x Pair Skin Grip Thumb Grip
1x Pair CQC Elite Thumb Grip
1x Pair FPS Master Thumb Grip Important Reminder: The Thumb Grip Set is non-capacitive.
Note: When you find thumb grips become greasy, please remove them and clean them with detergent.