I stand corrected.That doesn't quite track given . . .
While they haven't said exactly how Saw got them; there's basically two possibilities: 1) he stole them from Incom's mothball yards. 2) he used some of the cash from the Kessel heist (yes, he's the one Enfys Nest gave the coaxium to) to buy them off the black market.Saw already has a squadron of his Cavern Angels two or three years before the Alliance was even formed.
Either way, it's reasonably to presume that he's the reason the Alliance got X-Wings, not the other way around. Indeed they didn't show up in Rebels until very late on. Like, literally a few months prior to the events of Rogue One. At the top of season 4, the massassi group seemed to be still mostly flying Y-Wings.
I hate Star Wars sometimes.