Redfin.com and other MLS property listing places are worth looking at. Plus with these, if you want you can usually get legal permission from the seller.

Construction sites and rooftops are some of my favourites.

Sometimes, a site doesn't need to be abandoned to be worthwhile. I enjoy the challenge of reaching the rooftops of condos and skyscrapers. It's easy enough to get in and out, but then going from the top floor to the roof is always harder.

Rooftops hold a special place in my heart.

Douglas Hofstadter, somewhere in his big book about translation, talks about a game he played as a young'n, called "Leftmost Rightmost." You walk (or drive, or ride) down the street till you reach an intersection. You take, alternately, the leftmost then the rightmost option. (Sometimes straight might be "right," because it's your "most" right option.) That's it. It provides surprisingly satisfying randomness.

I play this game looking for locations. I like best: hotels, hospitals, malls, community centers, construction sites, buildings under renovation or asbestos abatement, office buildings, churches, city halls, cemeteries, libraries, airports, box stores, public buildings of any sort, parkades, and abandoned buildings. Especially hotels.

When I am driving around I always keep an eye out for anything that looks overgrown or boarded up. My area has plenty of abandoned buildings, most are old stores or businesses that are to small or uninteresting to bother with. The old style brick smokestacks are a good thing to look out for, they usually are visible a good distance away. I do a lot of searching online as well when I am bored at work.

My thing is old ranch houses and other rural stuff. I use google earth, and look for signs of dilapidation.