I love your question...check out the "bibbity bobbity boutique" in Fantasyland...costumes galore! Infant size, unsure...but they had a size for my DD when she was 3 yrs old...just last Christmas.
But what we did...when we were "at home" we picked up costumes at our local Toys'r'Us and toy store and stuffed them in the suitcase and viola...save yourself some SERIOUS cash!
My DD loved "discovering" her Minnie costume in our hotel closet!
Yes, they have them in infant sizes. We got an 18 month size for my niece last spring. They had them in most of the shops that sell clothes, not just Bibbity Bobbity Boutique. I also noticed that they carry them in the Disney store at the mall for much cheaper!
She look so cute in it