J.C.
Toys R Us has a little section, usually on an end cap in the "learning toys" section (near the Thomas and Melissa and Doug type stuff) with occupational costumes. They keep a handful of them in stock year-round. They are hanging and they're in plastic sleeves. They usually have fireman, doctor, conductor, that type of thing.
You could also buy the Fisher Price doctor kit and just use that with an oversized white shirt.