Ralph Martins cactus finder database query and Christophe Ludwigs database query are the most comprehensive tools available for finding cacti and other succulents.† You can search by species or location and has been the basis of my recent travels.† Data with km references are the most accurate.
found a reference to a location are you sure itís the right place? eg
Go to Geonames there
Google Earth map your planned travels and access roads, nearest accommodation, etc.
For a list of literature and synonyms referring to your target species go to IPNI
There are many home pages which have distribution maps for specific genera/species.† Go to the cactus mall home pages tab for links to these.
The archives of cacti_etc contain locations including some GPS.† You can search these and the results are emailed to you.† Or you can ask the forum.
Englera 16 ,† Eggli Schick & Leuenberger has over 600 pages of Ritters collection data.† Invaluable for South American cacti.
Copiapoa , Schulz & Kapitany has GPS references, maps and photographs of Copiapoa.
Gymnocalycium,† Charles has maps with location data and photographs for the entire genus.
Uebelmannia in their Environment, Schulz and Machado has GPS data, maps and photographs for this genus.
Journals The CSSA journal and Cactus & Co, to name just 2, have quite detailed locations as do journals in other languages.