Gathering "live" research and information for a magazine article can be one of the greatest challenges of writing. Some people simply don't want to be quoted on the record, whether for personal reasons or professional ones. They prefer to stay under the radar. Others ... well, you can't shut them up after they warm up to the idea and open their mouths.
Seize the Moment
Asking anyone to talk to you can be a matter of opportunity, even if you don't have an assignment in place yet. If someone's kitten was just rescued from a burning building and you happen to be right there on scene, jump in with both feet while you have the opportunity. Now you have an article you can pitch to a publisher. Sometimes the right interviewee will land you an assignment, so you'll want to include mention of him in your query. Just make sure that the kitty-owner knows that you're going to write about her experience and don't forget to get contact information for follow-up.
Pitching Without a Source
Can you just mention a possible source in your query, something along the lines of, "I plan to interview John Doe, the owner of the burning building?" I don't think so. Your query should not be full of what ifs and maybes. If you think the query is more likely to be accepted with the right source or expert named, then by all means call that expert and speak with him ahead of time. Tell him the subject and name some of the potential publishers you're planning to submit to. Be sure to give him a full range of possible websites, newspapers or magazines, not just the top names. Many experts are happy to talk about their subject matter, so don't be shy!
If Your Pitch Doesn't Really Need a Source
On the other hand, if having an expert lined up isn't necessarily something that's going to boost your query, save it for later. It's always easiest to go to your expert with an assignment already in hand – it lends credibility and gets his attention. Not only that, but you'll probably feel more confident as well.
The Bottom Line
The bottom line in this career is making sure those assignments continue to roll in, so keep that in mind as you slog through the getting started phase. Each assignment will be different and there's no one-size-fits-all rule. It depends to a great extent on what you're writing and to whom you're submitting a query. Do your research to find out what various publishers and websites expect in the way of pitches. You might find clear-cut directives right there, telling you not to bother if you don't have a source lined up. This is one of those professions where you'll be called upon to think on your feet – a lot. Analyze your needs each assignment or idea, then plan the best approach.