WhitePages.com, YellowPages.com, and 411.com are all free services that allow a user to search for mailing addresses by first or last name, plus city, state or zip code. Use common sense and independent verification before using information found at these sites.
A listing for one person may have several addresses. You may check the information for free by using the reverse address lookup tool on any of these sites. Enter one of the addresses from the relevant profile, then search to see if the correct profile name is associated with the address.
When browsing profiles on people search websites, be careful not to click any links labeled "sponsored." Information through those links is available only for a fee, and it is easy to accidentally make a purchase.
When considering the reliability of information found on these websites, note that YellowPages.com and WhitePages.com each have a clear way of letting someone claim a profile, plus a verification process if the claimant wants to edit the available information, while 411.com does not have such a transparent verification process.
If you have most of the needed address, but are still missing the zip code, USPS.com has a zip code lookup tool. Find it in the menu under "Mail & Ship."