If you switch to the F6/D2 series eyepiece, you can use the DK-19, which is available new. Alternatively, check KEH. They should still have my old DK-2 that I sold with my F5 last year.
I gave up trying to find one, got a thick slightly undersized rubber O-ring. Used a min grinder and slowly worked size down till it would fit. It was snug enough to have to be stretched, works prefect.