Get to know that person first. After knowing them I'm sure exchanging phone numbers will be a normal thing to do. If you're talking as more than friends and maybe only seeing this person once or twice, talk to them and maybe offer your number to them and they'll reciprocate.
Oh god, don't look in the phone book, that would be really stalkerish. I'd just ask someone for their number. As in "We should catch up some time, what's your number?"
If they refuse, just leave it at that. I once had a lesbian mistake me for a butch and demand my number. I had no idea how to tell her not only am I a man, I'm gay. So. Awkward. D;