Are you talking about little flannel bags? They usually come with new shoes for free. Or do you mean "condoms for shoes", rubber thingies to pull over your shoes in the rain? They are called "galoshes" and a search on google should come up with plenty of entries.
I am talking about just regular dust cover bags, like the ones that came with my Bruno Maglis. I have a pair of Antonio Mauritzis that I want to get some dust covers for. I would think you could pick them up somewhere.