The edition and translation of the Nirukta (and Nighaṇṭu) by Lakshman Sarup was originally published in four parts: Introduction; English translation; Sanskrit text; Indices and Appendices. The Motilal Banarsidass reprints include only the first three of these, despite their subtitle. The fourth volume, of Indices and Appendices, is the part that makes this text useful for reference purposes. Not only does it allow one to find a particular word in the Nirukta (or Nighaṇṭu), it also serves as a glossary, since it gives English equivalents throughout. Your query has spurred me on to scan my copy of this necessary volume and post it with the Sanskrit texts here: http://prajnaquest.fr/blog/sanskrit-texts-3/sanskrit-hindu-texts/