I found it on Ebay. There was lengthy thread about F100 and that ring here, back in the day. Nikon Canada asked for some ridiculous amount to fix it, something like $350, 5 or so years ago. My regular camera tech could not do it. Then I found frendly Nikon service and they replaced it in less than 10 minutes. It's shame how cheaply Nikon designed that ring. Probably, for 0.53$ more they could install proper metal ring with button to disengage tab for non Ai lenses