If you have our free search and been unsuccessful at finding your former co-worker's email there are a couple of things you may want to try next. Try looking for a staff page on your former company's website. This isn't going to work if have left the company, got fired, or changed emails, but it is the most logical place to start.
If that does not work you may want to pair their name and location in a search engine inquiry. Sometimes this can bring back a social network profile. You can also just browse specific sites with their name. LinkedIn and Facebook are good bets. A deep web "scrape" search with sites like Pipl.com could help too.
From here, if all that doesn't work, you will need to visit a third party lookup service. These services do have their down sides, like the fees they often charge, but they can also prove to be well worth it when you consider the time you save in searching. For a list of some of these services see our search guide here.
The other thing that seems to work if the situation does not call for immediate results is to have the email find you. You can drop the person's name on a webpage and wait for Google or some other search engine to index it. Add in some key words like: search, find, lookup, add "their name" (obviously), then another phrase like "looking for 'their name' email address", or just "their name" email. Chances are pretty good they will "Google" their name and find you page and respond.