Locator agents don't differ all that much from conventional mission agents and you find them using the same methods you would use to find a new mission agent.

Locator agents will locate people for you in exchange for ISK. Each locator agent has a time constraint on how often they can be used and the distance at which the agent can locate people varies with the agents level. Lower level locator agents are limited in range, while higher level agents can locate a people anywhere. High level agents also have a shorter timer before you can use their services again. The timer varies between roughly 15 minutes and an hour.

The location information should arrive in your EVE-mail inbox within 10 minutes. It looks something like this:

2010.05.26 10:42

The sleazebag is currently at Nourvukaiken II – Moon 1 – Modern Finances Investment Bank station in the Nourvukaiken system, Okela constellation of Lonetrek region.

The easiest way to find a locator agent out of game is using the EVE-Online Agent Finder