Source for Speedometer correction box?

[y2kimmel]

I'm finally going to do the differential upgrade on my E420. I found a great source for my differential, only $470 delivered. It's from a 1992 500SL with only 50k miles (I know - it's only a 2.65 instead of a 2.82, but I'm a wuss).

Now I need to find a black box to correct the signal to my speedometer. Does anyone have a source for this, and more importantly, any instructions for doing it?

Thanks,
Troy K.
1994 E420 84k