Formerly at the FTP site ftp.gmd.de, it is now at the web site http://www.ifarchive.org/. The Archive is also still accessible through FTP, at ftp://ftp.ifarchive.org/if-archive/. You might also like to consider using one of the Archive's mirror sites, which are listed on the Archive's home page: depending on where you are, you might well find a mirror that responds faster than the main Archive site.
The Archive's main collection of Glulx games is in the games/glulx directory. Another source of Glulx games is the annual IF Competition. Two other sites well worth a look are Baf's Guide to the Archive and Mike Robert's Interactive Fiction Database, both of which provide information and reviews of games in the IF Archive.