Sources can make or break your article. If they are friendly and helpful and answer their phone or email when they say they will, then writing can be a pleasant experience. But if they offer to do an interview and then go MIA or if they ramble incoherently about topics that are outside the scope of your article, it can be downright painful.
I’m still mastering the art of the interview, but I have a few go-to places for sources (and I rarely rely on just one because I don't want to be stuck without a source). Being a child of the computer age, I do almost all of my source searches online at these sites:
MediaBistro has a bulletin board devoted to finding sources. I've used it successfully, but remember that it's a site geared towards the media so you generally get PR people, freelancers, etc. If you're looking for a variety of "real people sources," you might want to diversify. Click the box for allowing email so your sources can contact you personally and get a dialogue going away from the boards.
WritersWeekly also has a forum for those who need sources. Again, it's mostly writers who use the board, but I generally get a good response rate. If you're on a tight deadline, keep in mind that the new edition goes live every Wednesday. Absolute Write also has as section of their forums for research.
Facebook has a group called "If I can help a reporter out, I will... " and the moderator (Peter Shankman) sends out messages whenever someone needs an expert on estate law, a "real person" who's flown around the world, or any other variation. Peter would love to add more users to the group, too. I've also used facebook on my own. Posting notes about source needs hasn't worked too well for me, so now I do an advanced search through all my facebook friends to find people who live in different parts of the country, attend grad school, practice yoga, etc.
LinkedIn has a section for questions and answers. I haven't used it yet myself (I know shame on me for not practicing what I preach), but other people use it a lot and since LinkedIn has a cross-section from many different industries, I could see it working very successfully, although you might get many more responses than you could ever use, which is also a problem with ProfNet.
For more on finding sources and conducting interviews, check out these posts on Ask Allison and Writing White Papers.