If you mean just the discs that are red/blue for the hot and cold, then I don't think you can buy them. They are not all the same size for one thing. Someone asked this question a while back and the answer was to go into a DIY store and flip them off the display taps!!
If you mean the complete plastic heads like you have on kitchen taps, you can but them from B&Q (that's where I got mine from) and probably other places as well.