I don't know what the specific walker.[br]
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But often things with hard plastic wheels use standard ball barrings that's also used on skateboards, rollerskates and scooters.[br]
A good place to buy new bearings is a skate shop, since they will usually be in the correct size, and of good quality.
The hardest part is getting the old bearings out.
[link|https://www.rollerskatedad.com/how-to-remove-bearings-from-roller-skate-wheels/|Heres a guide for doing it on roller skates|new_window=true] but if you look at method 3, that should be applicable to all things using similar bearings