WordNet (r) 3.0 (2006):

light 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]

Moby Thesaurus II by Grady Ward, 1.0:

43 Moby Thesaurus words for "light upon":
   alight upon, arrive at, arrive upon, blunder upon, bump into,
   chance upon, come across, come among, come at, come down on,
   come on, come up against, come upon, confront, descend upon,
   discover serendipitously, drop on, encounter, fall across,
   fall among, fall foul of, fall in with, fall on, fall upon, get at,
   happen upon, hit upon, meet, meet head-on, meet up with, meet with,
   pitch upon, reach, run across, run into, run smack into,
   run up against, run upon, settle on, strike upon, stumble on,
   stumble upon, tumble on