Try searching the area or city you live in and parkour. So "New York parkour" for example. Do this on both google and on facebook and there's a chance you can find a local group or gym to meet up with. Another option you have is to do the same thing but search for gymnastics gyms and find one with open gym hours. If you go train there there's an ok chance that you'll meet a group of free runners as well. That's actually how I met the guys I train with.
If those don't work out, then I second the parks idea. Playgrounds and parks usually have a bunch of obstacles. Aside from that just explore. Look for low walls, ledges, rails, and things of that nature. Anything you can climb, jump, or vault and have at it.