1. Go to the bookstore and find books similar to yours. Look in the front on the dedication page. A lot of times, authors will thank their agents. Query them.
2. Find a local library which subscribes to Publisher's Weekly. Scan through each issue and note the comings and goings of agents. An agent who has left an established agency and gone out on her own probably will be looking for new writers.
3. See if your library carries the Literary Market Place. It's a big book that comes out twice a year, I believe. Among other things, it lists Agents. The nice thing about that is that it lists what kind of material the agency is interested in looking at. There's no point in querying an agency with your nonfiction book proposal if he only reps gothic romance!
4. Of course, a really good way to find a reputable, "brave" agent is to get a friend to recommend you to hers. If you don't know other authors, then join a writers group and make friends. 'Course, that's sort of a long-term plan, since you can't just walk up to strangers and ask who their agents are. You have to build a relationship with them.
5. Go to conferences and workshops and meet other writers. You can also meet agents and editors there. When you meet an agent you like, you can ask if he'll accept a query from you. If he says yes, then send it to him, along with a reminder of where you met.
6. Go to some of the major search engines, such as Yahoo, Google, HotBot, etc., and type in keywords like Agent or Literary Agencies or Author Representatives, and see what they bring up. Bookmark agencies that look interesting, then do some research on them.
Keep in mind, though, that whether you end up with a well-known agent or an up-and-coming one, you'll still be doing a lot of your own marketing. The Grishams, Rowlings, Kings, Graftons, and the like may have people who do the marketing and arrange PR, but most everyone else does a lot of their own, to varying degree depending on how much is set aside by the publisher for marketing.
Sometimes we forget that while agents can be elusive, they are always human (contrary to rumor). They make wise decisions and they make mistakes. If an agent rejects your idea or manuscript, don't assume it's bad and will never sell. Move on to the next agent on your list. And the next and the next. Refine your query letter; edit if you feel it's needed. Write new material. Work your strategy. Go out and meet other authors, attend conferences, research new agents.
Just as we all hope to find an agent brave enough to take on a new author, be brave yourself. Giving up is taking the easy way out. Toughing it out and continuing against odds takes courage. Sure, some day you may decide to give up on a particular manuscript, but don't let an agent, friend, co-worker, critique partner, or your mother tell you to do so.