WordNet (r) 3.0 (2006):

come upon
    v 1: find unexpectedly; "the archeologists chanced upon an old
         tomb"; "she struck a goldmine"; "The hikers finally struck
         the main path to the lake" [syn: fall upon, strike,
         come upon, light upon, chance upon, come across,
         chance on, happen upon, attain, discover]
    2: take possession of; "She entered upon the estate of her rich
       relatives" [syn: enter upon, come upon, luck into]

Moby Thesaurus II by Grady Ward, 1.0:

47 Moby Thesaurus words for "come upon":
   alight upon, appear unexpectedly, arrive at, arrive upon,
   be unexpected, blunder upon, bob up, bump into, chance upon,
   come across, come among, come at, come on, come unawares,
   come up against, confront, creep up on, discover serendipitously,
   encounter, fall across, fall among, fall foul of, fall in with,
   fall upon, flash upon one, get at, happen upon, hit upon,
   light upon, meet, meet head-on, meet up with, meet with,
   pitch upon, pop up, pounce upon, reach, run across, run into,
   run smack into, run up against, run upon, strike upon, stumble on,
   stumble upon, tumble on, turn up