You can now enable Cirrus as a beta feature on all wikis on which Cirrus is a secondary.  On English wikisource the link is https://en.wikisource.org/wiki/Special:Preferences#mw-prefsection-betafeatures.  The name of the feature is "New search" for now.  Yup, we know that it won't be "new" for too long but we hope that it'll become the standard search before it stops being "new".