Are you wedded to the idea of a "real" Minnie costume? I ask because if you haunt consignment/thrift stores, I'd bet you could find a polka-dot dress. Add black tights, a mouse headband (I got mine at DTD) and cover a pair of shoes with yellow duct-tape, and you're done.
FWIW, that's what I did for my daughter this past year, and it turned out quite cute!