How do I find a replacement nylon shower door runner / wheel?

I have a nylon runner / wheel for a shower door but do not know the make (like many people) as it was fitted by a builder. It sits at the bottom of the shower door to keep it on track while the weight is carried at the top of the door. It is one of two runners.

The wheel is nylon on the outside and metal on the inside. The metal has corroded very badly and the nylon has broken. It would appear to be 20mm maximum diameter with a 9 mm diameter hole in the metal insert and 6 mm in thickness.

Any advice of where I might be able to get a replacement would be greatly appreciated! Many thanks.