Taking wing
The oceans of information at our fingertips can daunt as well as inspire us as we search for viable freelancing ideas. I would say don't overlook intuition and serendipity as you browse and brainstorm.
An idea-stimulating annual volume to peruse is The Writer's Market. Even old editions of this big reference book still render dozens of relevant categories of publications and publishing-related work and lots of actually useful advice. Sections my journalism students seemed to like were How much do I charge? for various gigs and How to write query letters. One student right away went out and sold a photo-story to a national fishing magazine for $600.
New, small, niche finds help, too. The Geekpreneur site focuses on "geek culture" and how to "make money being a geek," yet plenty of the material applies to nontech freelancers as well.
Every field has associations whose websites give free advice, connect members to promising contacts, and post yet more links in their respective lines of work.
Surfing can lead to exciting discoveries -- and then the day comes to submit your first, carefully crafted, correctly spelled, and factually correct query pitching what you, the freelancer-to-be, can offer a specific potential client whose business you've researched and whose needs you could meet.
May I say this: Don't lose heart when you hear "No, thanks." We all get turned down some. Stay cool and learn what you can, metabolize and move on. Soon enough, the rush of hearing "OK" will energize you, and the rewards of your first project will reassure you that freelancing can be fun. Good luck, and enjoy the trip!