@ Evolution, Google will often give you the info you need, do a search for:
"Registrant: their name here" (include the quotes)
Alternatively, if they are a tag holder, do a search for their tag in the form of:
"[Tag = THEIRTAGHERE]" (include the quotes)
Then it's just a case of going through all the domains in the search results owned by that person, until you get lucky and find one where they have used their address in the registration, or a live website they own that has their current contact address/phone/email on it.
Also, on the domain you are looking for, take a look at the source code of their website, that sometimes gives details or a link to another of their sites, possibly opening a frame from the current site, as was the case when I found the contact details for someone on here the other day where their site opened in a frame from a .com site registered to them, which included all their contact details.