I got Monster High Catty Noir and seeing her boots got me thinking that I might be able to copy them. The boots consist of two halves, which should make it possible to make tubular latex moulds for them.
I attached the parts to small pieces of cardboard covered with packing tape. I used clear mounting tape to attach the pieces from the toe parts and them applied a few coats of latex. I had to make moulds for all four parts, because the two halves of one boot were different and the corresponding parts for the other boot were mirror images of those.
Here are two of the finished moulds. I was a bit worried that the holes wouldn't be big enough to get the pieces out of the moulds, but latex stretches quite a lot, so the moulds were usable.
Here you can see the size of the holes. The biggest problem with them came when pouring resin into the moulds as it would easily spill outside the mould.
Making the castings wasn't as straight forward as I had thought, because hanging the moulds vertically caused the moulds to deform as resin collected to the bottom. I had to fill the moulds half full first and place them almost horizontally so they would keep their form. Then when the resin had hardered a bit, I could pour in the rest and keep the moulds upright to keep the resin from spilling out.
Here you can see the original pair and the resin castings. The castings aren't quite successful as the seam in front won't close properly, but the result is still better than I expected. I'll just need to figure out a way to fix the front.