Digital Map Indexes. Every index map held in the collection of the Earth Sciences and Map Library at UC Berkeley.
Index Map Collection. Western Michigan University. Primarily AMS or GSGS map series.
The British Library. Search their Integrated Catalog for "Index to series attached" as a phrase to find all map series having digital index maps (primarily GSGS series (Geographical Section, General Staff)). Choose a set of interest. The scanned index is available from that record.