Abebooks, Mercado Libre (LatAm), or Iberlibro are excellent for out-of-print Spanish books. Search for the ISBN if you have it.

While you hunt for the elusive Helena PDF, here are thematically related works that are legally available in digital format:

If you enjoy poetic enigmas, Borges is your closest bet. His story La Biblioteca de Babel shares the same claustrophobic, puzzle-like atmosphere attributed to Helena .