I found him a black coat at the Goodwill outlet (I only paid a little over a dollar for it). I was inspired by this costume on sale on Etsy for the hat. I found a black hat at the dollar store and sewed on scraps of fabric and ribbon that I had on hand. Pretty easy, and cheap!
Here's a better shot of the hat:
In case you missed them, you can see my son's bat costume here, my cat costume here, and come back tomorrow for my baby's robin costume.
PS Kathryn guessed our theme, batman, but with out the 'man' (or woman). :)