These are handy for the summer, to say the least: the HBO store now has Game of Thrones caps (at present featuring the logo, plus three of the major houses). You can get over to them here, or check out the Westeros.org store!
And, by the by, look at the top of the HBO Store: a 20% sale on all items, not just Game of Thrones, with the use of the SAVE20 code.