I have a Nikon F4s with the MB-21 battery pack. It works perfectly but I don't need the extra speed for shooting sports etc.
I would like to have an MB-20 that comes on the standard F4.
MB-20s are hard to find. I wanted one for years and had little luck, and when I did see them they were a couple of hundred dollars.
I finally solved the problem by buying an F4/MB-20 from KEH and buying an MB-21 so I could sell the F4 and MB-21 as an F4S. (Funny though, I never did sell it. )
Camera Clinic in Shoreline, Washington may have one. Give them a call between 1pm and 5 pm your time at 206-367-2440. Guy's name is Doug. Used to belong to me, and, prior to me, to Fred (flatulent1).