I also disregard T shirt to my collection as its hard to identify which one is official or not, otherwise its from the official site. But I've got a few official Assassin's Creed T shirt from
http://store.ubiworkshop.com/ , They also have jackets/hoodie which are interesting like the Desmond Hoodie, etc.
Maybe my only favorite T shirt for AC is the Ace of Spade which apparently a prize a won from the minigame
