I've just spent the last hour researching a textile factory in Texas that I stumbled across by chance. Despite even knowing its location and name, my results have been pretty scarce, save for a couple old newspaper articles about the plant's eventual bankruptcy and subsequent dissolution via the auction block.
I guess I say all of that just to say, many (if not most) of the best locations I've explored, have had little or no online information on them. This holds especially true for the ones that have not been added to any known DB, UER or otherwise. The most fruitful locations I've come across, with one huge exception, have been by dumb luck or spending more time than I care to admit on Google Earth. It takes time, but I promise, your effort will pay off if you keep at it. Having a pathological map obsession like I do doesn't hurt your chances either.