It's fanon - something that is repeated so much in fanfiction that it basically becomes canon for fanfic.

I mean, it makes sense - Sam's the one who likes hoodies. We never see Dean in hoodies, so it makes sense that Sam would be the one that buys the hoodies between the two of them. Dean likes button down shirts, and has considerably less clothes than Sam. I can't really picture him buying a hoodie that he barely uses.

Anyway, I always thought that it was just a fanon thing with absolutely no evidence...because Sam likes brown hoodies, and the hoodie in Faith was charcoal black, but, I stand corrected. It's either that, or Sam borrowed DEAN's hoodie :P