WordNet (r) 3.0 (2006):

by choice
    adv 1: with intention; in an intentional manner; "he used that
           word intentionally"; "I did this by choice" [syn:
           intentionally, deliberately, designedly, on
           purpose, purposely, advisedly, by choice, by
           design] [ant: accidentally, by chance,
           circumstantially, unexpectedly, unintentionally]

Moby Thesaurus II by Grady Ward, 1.0:

19 Moby Thesaurus words for "by choice":
   before, by election, by vote, electively, first, freely,
   gratuitously, in preference, independently, optionally, preferably,
   rather, rather than, sooner, sooner than, spontaneously,
   voluntarily, willfully, without coercion