1
general::
Phrase(s): stumble across someone or something [and] stumble (up)on someone or something; stumble into someone or something
to find someone or something, usually by accident. (See also stumble into someone or something.) • I stumbled across an interesting book yesterday when I was shopping. • Guess who I stumbled into at the library yesterday? • I got lucky and stumbled on the right answer.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
Also, stumble on. Find by chance, discover or meet with unexpectedly. For example, When we were hiking up the mountain we stumbled across a few abandoned shepherd's huts, or At the flea market Alfred stumbled on a quite valuable old lithograph. This idiom uses stumble in the sense of "accidentally trip." [Mid-1500s]
American Heritage Idioms