The Collaborative International Dictionary of English v.0.48:

Availableness \A*vail"a*ble*ness\, n.
   1. Competent power; validity; efficacy; as, the availableness
      of a title. [Obs.]
      [1913 Webster]

   2. Quality of being available; capability of being used for
      the purpose intended. --Sir M. Hale.
      [1913 Webster]

WordNet (r) 3.0 (2006):

availableness
    n 1: the quality of being at hand when needed [syn: handiness,
         accessibility, availability, availableness] [ant:
         inaccessibility, unavailability]