The Map Catalogue can be accessed here. It is in the form of an Excel spreadsheet, and a search can be carried out as for any Excel spreadsheet.
Ctl + F will open up the Find and Replace box, and the specific name of a map or, if not known, a locality, can be entered. A subject search can be carried out in the same way – e.g. Geology, Soils, etc. Lengthy lists may be produced, and it may be best for a search to be conducted through the country (where known) or region, and then working through the list for the subject.
An alternative is to check map indexes here for country and scale required (mostly topographical or geological) and see if the area required is held in the Map Room.