As a goth girl in America going to the Hot Topic can be a little cliche but there's a seemingly endless array of GIR shirts, Portal toys and Batman socks that call me in time and again. This Summer they've added some very cute summer dresses for the Nerd girl at large (and on the beach!)
There are even more to be had including R2D2, Power Rangers, TMNT, Captain America and Toy Story. At the bargain price of $15 - 35 these could be the easiest convention outfit you find.