I found matching hard floor castors at Toptip (I had taken a carpet castor for comparison). Fit. I think they are universal sizes.

The Topstar hard floor castors from Digitec fit (note: it must be the version with 11mm pins).

Castors like these (11mm) also fit: L33T castors for gaming chairs found at Accessories Gaming. I have these for the wooden parquet - runs well and red/pink castors also have that certain optical something ;-)