well ..... in my opinion it's someone unknown (we haven't seen before) and the fact that he said i had an informant more than once shows how much jiraiya has been around (an old shinobi with a lot of experience) .... but to answer from your list :
- itachi (I can't see a single reason why itachi would help jiraiya, being a good guy didn't stop him from torturing kakashi and attempting to kill the others, he did one good deed in his life and that was killing the uchiha to save the village) thats why we all love itachi

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- obito (would be my answer coz he has a strong motive, ELEMINATING THE STRONGEST SHINOBI IN KONOHA AT THAT TIME so the plan can go more smoothly and nagato is a monster who knows his master better than anyone).
- orochimaru (hmmm .... just no

, the last thing oro would do is help jiraiya besides destroying konoha is kind of his goal and i don't think who destroys it matters to him plus jiraiya wouldn't trust any info from oro and would attempt to kill him on sight).
- kabuto (looking at how much he knew about tobi and the plan, he wouldn't do something that can risk obito's plan since he was preparing, researching all to join obito, also kabuto wouldn't do something that doesn't benefit him and to help orochimaru sama's rival and enemy .... no i don't think he would ever do that).
so my answer from the characters you mentioned would be obito, but am sure its someone we don't know coz jiraiya wouldn't trust any akatsuki's info