I had a customer's D90 in recently with 1 knob missing. I wasn't able to find an identical replacement, but did source a aluminum-over-plastic knob that fit the stem.
They are larger in diameter and aluminum instead of black, and I had to order 12 to get a minimum order amount, so I'd be glad to send you a pair.
I'll post a pic when I get out to the shop.
Roverlab is now British 4x4 Specialists.
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Same place, new dogs.