There are a number of different ways you can find people to interview, but one of the best is really to pay attention to the publications and other media you track in your market segment. Specifically, who gets quoted when articles or blog entries are written about your specific product or service area? Those are some of the very best people to approach for an interview because they're already recognized market authorities.
You can also use social media tools to spread the word that you seek an expert or thought leader in a given segment. For example, if you have a number of followers already, you might send out a request on Twitter asking for a connection to someone. LinkedIn and FaceBook also offer you the ability to search by profession or keyword and thereby identify candidates with whom you are ostensibly connected already.
Another service that might prove quite valuable for you is Help A Reporter where, without charge, you can send out a request for interview candidates for your blog. The readership (over 25,000!) is comprised of both publicists, public relations experts and community members. The results can be astonishing.
Here's an example I just experienced myself. Seeking one or two people who were developing applications for the Apple iPhone, I submitted the following query:
Summary: Want to interview iPhone app developers
Category: Technology
Name: Dave Taylor
Email: d1taylor@gmail.com
Title: Industry Analyst, Blogger
Media Outlet/Publication: The Business Blog @ Intuitive.com
"I'm pulling together an interview for my popular business and tech blog about the experience of writing an app and getting it into the iPhone Application Store. I already have one and would like to extend it out to a few more application programmers who have a game/utility/tool currently online and for sale (or for free) in the Apple iPhone App store. It's a set of questions already written: I'll email 'em to you, you answer them, send me a link to your app in the store, and I'll help you get some publicity.
And don't worry, there's nothing that will violate the NDA you have to sign with Apple to get the SDK."
Not an overly complicated query, you can see that it's honest and explains exactly what I seek.
Within four hours my Gmail inbox was stuffed full of responses and less than eight hours later, I have over thirty iPhone application developers who not only are volunteering to be interviewed but want to offer books they're writing, new apps they're developing and various other services to me in return for some additional visibility for their projects.
Nice. And the cost? Just the time to respond politely and professionally to each.
There are indeed many different ways to track down people to interview and many different ways to interview them too: some people are strong proponents of phone-based interviews, while others - myself included - are believers in email-based question and answer. The best solution in that regard is undoubtedly to ask your interview subject what they'd prefer: treat them with respect and you'll get a far better and more interesting result.
Good luck to you.
Additional links: How to interview people for your blog, my Twitter account, Help A Reporter, and, finally, free iPhone help.
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When I saw the title of this entry, I was thinking "HARO (Help A Reporter Out) would work", only to see that that was your already your answer.
I used it to put out a query for our startup blog / membership site about spyware, and received numerous responses, including someone from Webroot and another with a contact at Symantec.
I heartily agree with Dave's answer on this one. What have you got to lose? (Well, unless you run into the "problem" some posters run into -- too many good replies.)