Just address any mail to International Space Station number 9, Space. I think there's only one of them (or maybe the first 8 are still orbiting, I'm not sure.)
factor-fiction it seems you can email them and follow them on twitter (NASA_Astronauts is the twitter account they all use) but I have no idea how you find the email addresses of the astronauts.
Post/zip codes are there to assist automated sorting systems. Since the amount of mail heading for the ISS must be minimal, and because the ISS definitely isn't on any postie's 'walk', there's no logical reason or it to have one.
However the USPS might decide to allocate a zip code to the ISS for purely symbolic reasons (in much the same way that the Russians have just taken an unlit Olympic torch into space for purely symbolic reasons).