Yes, I know this is an old post, but I just discovered it while looking for Mary Poppins costume ideas.
DD wants to dress like MP for our next trip. I don't sew very well, but I am creative. I found a used wedding dress in her size, that is very similar to MPs dress, for $15.00. I took off the puffies off the sleeves to make them like MPs and shortened the length and sleeves. I took off the appliques, took the sparkles off them, and will now reattatch a few to the dress, just like Mary's. The dress does not have a chiffon overlay. A friend is making the cumberbund/belt. The jabot was reasonably easy enough but the spats are a challenge. I made them from the train of the dress, so they would be the same fabric, but now I'm thinking I should go to a hardware store and get 1/2 yd of white plastic so they look more like leather. We'll see. I bought a second hand hat the right shape, but I had to spray paint it white. If I could find a white umbrella, I'd try painting sections or using a fabric overlay, but at this point I can't get a white one, let alone a cheap white one. Anyway, it's a work in progress, but I'm having fun, and DD is really looking forward to wearing it. She already has a white crinoline so it should look pretty good.