I may have a complete set here if you need them.

I'll clarify though, it's half urethane and half rubber. From my mini days, where their castor rods aren't the hardest steel around, it's actually possible for full urethane bushes to tear up the castor rod eventually - no kidding. The 'fix' was simply to run half and half, the urethane for the front halves, which are the ones that wear under heavy braking etc, and rubber at the rear. That combo has all the firmness and lack of slop that urethane provides, with just enough compliance to keep it all happy.

I swear by it. Anyhoo, as a result of that, I've still got the other 'halves' of each since I bought 2 sets to make em.

for anyone else who might be curious, you can get the rubber ones for about $5 each, the urethane ones are a bit dearer.