Whenever you want or have time to start searching. Mine doesn't sign contracts until after the first trimester, but other than that, she has no restrictions. I know mine already and have seen her in action at friends' births, so I know we are OK personality wise, so I guess I'll just go through what I'm looking for in a birth, have her meet my husband and answer his questions, that kind of thing. You could ask about experience, certification, what she brings to a birth for supplies, when she'll come in labor, if she's familiar with a particular childbirth method (if you care), contract terms, backup in case she's ill. If she relies on massage, but you hate being touched, that sort of thing would be good to know. Saw an interesting thing on another board mentioning you might want to ask about religion being incorporated into the doula practice - maybe you want that, or maybe it would drive you nuts.
& always ask for references. good luck!