In many of the robots, we would use the LEGO Steel Ball so you should try to acquire one. Even better if you have two of those. One is available in the LEGO Education EV3 core set 45544, but the LEGO EV3 Retail Version 31313 does not have them.
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- 05 Dec 2017
Where to buy them from?
You can buy them, or you could really be creative when replacing them with other third wheels. It is your choice. If you decide to buy them there are a number of places. The set is called "LME Replacement Pack 3 2000702" so a quick internet search should find you some good placed to buy them from. Generally, LEGO should have them on their site. Below you can find a picture of the Steel Ball and here are a few links that you could use