Where can I get just a replacement acrylic tap handle?

Where can I get just a replacement acrylic tap handle for a Hygena kitchen sink mixer tap (1970's) & not a complete tap ? (I've done a Google search). The tap handle has a square 1/4" (??) socket inside it which the tap spindle fits into. Any tap handles which fit would do - not necessarily a Hygena one.

I can only find a complete replacement tap. I bought a complete replacement tap & fitted it OK but it wouldn't turn the water off no matter how far I turned the tap off. I thought that any tap would function if the mounting thread size was correct ... Thanks.