Amazon.com: MOC Universal Wheel 92911 bb607 Stainles Steel Ball Compatible with Robot EV3 6023956 4610380 99948 Building Blocks Technical Parts Power Functions Pack of 4PCS : Toys & Games
Amazon.com: LEGO Technic Steel Ball Caster : Toys & Games
Amazon.com: MOC Universal Wheel 92911 bb607 Stainles Steel Ball Compatible with Robot EV3 6023956 4610380 99948 Building Blocks Technical Parts Power Functions Pack of 4PCS : Toys & Games
LEGO part 99948 - Technic, Ball, Steel (fits 92911) at BrickScout