There are two easy ways to find a person on Twitter, even when the person is not using her/his exact name as a Twitter ID.
First, you can use Twiangulate's "keyword" search for the person's name. Here are 53 people who are identifying themselves as "Lucy Smith" on Twitter.
If this method doesn't work, try to think of two or three people who might know the person you're looking for and then input their names into Twiangulate's "common friends" search.
For example, if you're looking to follow Elizabeth Spiers, Gawker's first editor, you might look for her among the friends of NYC media insiders like David Carr (@carr2n) and Gawker (@gawker) and Anil Dash (@anildash.) Sure enough, she's right there.