They just pull off but if they have broken you may have to remove the front panel to get at the remaining part on each spindle. There are screws at each end to enable the panel to come off.
The knobs (or what is left of them) will be tight and will require a firm pull, wriggling as you go.
I got my new knobs from espares (internet sales), they're not cheap, but I doubt you'll beat their prices and service. When you fit the new ones, which just push on, grease the spindles lightly to facilitate removal in the future.
Chicks Fan, January 2009
