I'm an 11.5 as well, which is unfortunate because half sizes are really rare above an 11. I can usually fit into an 11 though, especially if I'm not going to be on my feet all day.
I've ordered from Zappos several times and I've always been been very happy with their customer service. Sometimes items will ship the same day (!) and shipping is really fast. Turnaround times are really good on returns as well. I've put something in the mail as a return on Monday and had my new pair by Saturday.