Maps, library local history sections are newspapers are good and will often bring up locations that aren't on the Urbex tourist trail.
Internet searches tend to be more productive. Don't just settle for Google: try flickr, Geograph, etc. Search terms around the theme of 'Derelict/abandoned/disused + locality' are surprisingly effective.
I can't personally recommend driving around aimlessly looking for locations; it's a waste of fuel more often than not. Instead keep an eye out on ordinary car or train trips or take an unfamiliar route to somewhere you were going anyway - it saves fuel and it's worked for me plenty of times.