Sign up for a websitse dedicated to fostering language exchanges like Mylanguageexchange.com, Sharedtalk.com and Polyglot-learn-language.com just to name a few. Some require registration or a fee but many can be used free of charge. You can also use community portal sites like Craigslist.org in the "Activity Partners" section. Make a language profile or post a message seeking speakers of your target language. Of course, always offer your language skills to them as well.
Post bulletins at community colleges and universities, especially in larger cities with diverse student populations. Ask if you can post fliers on public bulletin boards and leave them up around campus. Include any languages you speak, as well as the language you're looking to learn or practice.
Ask friends and coworkers if they know anyone who speaks your target language. If they're interested, offer your language skills, or any other skill you've got in exchange. Post bulletins at work or any local community center or library.