What soft, cushion material can I use for slightly padding something and where can I get it?

You may want to look at cotton batting. It's the stuff you would put on top of a foam layer if you were making a padded head board of, even reupholstering a chair. If you have a local Walmart with a sewing dept. that sells fabrics, or any fabric shop would carry it. Go take a look and see if that would help you, as I'm not really sure if I'm getting it, as to what you want to do with your boots. I just read 1/2 inch thick and compresses like a cushion and thought of cotton batting. Check it out, ok?.