You can find it at http://sourceforge.net/projects/portableaserve, and the best way to load it into a project (as with most CL libraries these days) is via Quicklisp.
NOTE - Portable Allegroserve has been maintained with patches recently, for compatibility with LispWorks, SBCL/Linux, and CCL. At some point it could use a refactoring for acl-compat to use now-defacto-standard libraries from Quicklisp (e.g. Bordeaux-threads), and to track the current original AllegroServe. But the current Portable AllegroServe is usable at least on the platforms mentioned.