I ordered my sparkle skirt for this year off of
Amazon Prime, so that might be an option for you. If you get really stuck I see that both Sparkle Skirts and Sparkle Athletic have a booth at the expo so you could probably find something there. Tiaras you could find on Amazon too I am sure, or if you aren't too picky on how fancy you want a tiara I have seen some at Walmart or Claires. There is also a store at Disney Springs that sells tiaras and they are very pretty!
I tend to go with normal running clothes but theme them as a character (see avatar - last year I was Snow White) so I am still comfy but still in the spirit. I always have a sparkle skirt of some kind on though, covers my rear and it is shiny and pretty! This year I am wearing a purple shirt, green sparkle skirt and red headband to be in the theme of Ariel!