just go to MSG before a knicks game (there's game 2 tomorrow night). they usually have people who set up tshirt stands with all the good stuff. the best part is that the shirts are actually cheap (10-15bucks).
outside of TD garden and fenway before/after games you'll see a ton of novelty t-shirt sellers on the street (things like "I hate Peyton Manning.... and Eli Sucks too!") my personal favorite was the lebron parody shirts.