I have a feeling that they're not replaceable. If I recall correctly they're LEDs and soldered onto the instrument cluster circuit board.
 
The lights on the heater control panel on my 2001 are wedge base capless 12v/100mA jobbies (these are also the same as the instrument lights on a Disco 1), so I'd guess they're the same.
I could be wrong though on the freeby.

If they are WBC jobs, any half decent motor factor will have them... I got mine for the disco from halfrauds ;)
 

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