The Nikon literature (if you dig deeply) will reveal that there was never a -1 diopter made in that size, oddly enough. I used to purchase the -1 diopters from Voigtlander through Camera Quest or Photo Village, but those have dried up, it seems. Recently I needed another one for a new FE2, and finally wound up purchasing one through B&H as a special order. It is a Zeiss diopter (same as the Voigtlander), and they were made for the ZI and Zeiss' accessory finders. I don't know how long these will continue to be made, so I would recommend stocking up on a couple, even if they aren't cheap at about $30.00. I had absolutely no luck scouring ebay.
To somewhat contradict bluesun267's response, the Nikon finder is already at -1. My eye needs a -2.25 correction, so the -1 of the finder plus the -1 diopter lens gave me a -2, which is just about perfect. All terribly counterintuitive!