PostGis
You need to download and compile the source code from the PostGis web page: http://postgis.refractions.net/, but there is the need to have other working packages allready installed: GEOS and PROJ. You can download from the respective web pages.
PostGis on Mac OSX
You can download download and compile the source code from the PostGis web page: http://postgis.refractions.net/
For GEOS and PROJ, use the macports project: http://www.macports.org/
There are some italian instructions on this blog: Blog del consulente informatico