You'll have to give us a bit more info than that old son. What sort of attachment do they use? I can tell they're not collet type, but are they grubscrew or knurled? Do they have a brass insert or is it straight into the plastic? What's the diameter?
There's a lot of hardware sources out there that I've never seen mentioned on this or any other forum, so I'll keep my eyes peeled. [i]Someone[/i] will almost certainly have them, as I doubt EMMA do enough business to get custom knobs made, but they may well be NOS or obsolete parts, and therefore nigh on impossible to source.
Andy