How rare is it to find an anime script from the late 1990s for purchase?
Since I posted the question half a year ago, I've been tracking Sailor Moon scripts on Yahoo! Japan Auctions more regularly. If you are looking for a script of a specific episode, you should act when it becomes available, because there are always some scripts up for sale, but not scripts of most of the episodes of such a long series. At a given time, about 10~30 episode scripts are on auction. Scripts from the Sailor Stars season (season 5) rarely go up on auction (I've only seen 2 listed in the past year). The Japanese used (i.e. pre-owned) merchandise store called Mandarake (online shop, plus branches in Tokyo, Osaka, Nagoya, Sapporo, etc.) sells scripts from a variety of series.