Can't go wrong with the classic "Yankees Suck"...outside Fenway Park after home games (not just Yankee games) they sell various anti-Yankee items such as Flags, Shirts, Hats, etc. I believe they have a web site (try a google search for "Yankees Suck"), you could go there and I'm sure there a few slogans worth borrowing from there.
I'll personally be at Game 3 in full "Yankees Suck" apparel complete with "Yankees Suck" minature flag. I'll be the one that's crying every time Aaron Boone is announced....