"Traditionally, what is Matthew's source of information for the first few
chapters of his gospel? There's a bunch of content where he plainly
wasn't an eyewitness; do we know who his informants were?"
I haven't really looked up what others say, but since I believe Matthew was written by the disciple named Matthew/Levi who actually walked with Jesus for 3.5 years, it just seems natural he asked Jesus himself around the campfire at night at some point. It appears there were women who traveled with the group helping in the ministry, and Mary was likely one of those at least occasionally. So she might have told the story of the Wise Men's visit herself.
"As a tax collector, Matthew possessed a
skill that makes his writing all the more exciting for Christians. Tax
collectors were expected to be able to write in a form of shorthand,
which essentially meant that Matthew could record a person’s words as
they spoke, word for word. This ability means that the words of Matthew
are not only inspired by the Holy Spirit, but should represent an actual
transcript of some of Christ’s sermons. For example, the Sermon on the
Mount, as recorded in chapters 5-7, is almost certainly a perfect
recording of that great message."Source
Christianity also teaches that Mary was active in the church until her death (AD 41 ish? The only date I could find), so I would think it possible that Matthew could have asked her later, maybe as he was preparing to write his gospel.
By the way, there is a difference between what Catholics believe and other Christians. The catholic religion has added in a lot of stories over the centuries that have no basis in historical fact. These might be fun to read, but they are not truth. In fact I usually find them more on the order of fairy tales than Bible.