I second Nordstrom's, and if you have a local Nordstrom's Rack, maybe non-nonchalantly walk though the big heel section.

I'm a size 12, and I accidently bought a pair of Pleaser pumps in size 12 (I didn't understand that 12m wasn't 12 mens, but a length and width notation) and they're a bit snug... but totally wearable. I bought a pair of Pleaser pumps in size 14... which "should be right" and they're a bit too big. They don't fall off of my feet, but 13s would probably have been the sweet spot.

Find an online store with a robust and simple return policy, try them on in the house, send them back if they don't fit. I kept both pairs of mine, and like them both (they're ok in quality, but fun and sexy and not too expensive).