If you can find vinyl boots in the style you want, you can spraypaint them yellow with a spraypaint formulated for plastics. Scuff the surface with a scotchbrite pad or fine steel wool before painting, and apply 3 coats following the directions. You want to give the boots about a week to cure before touching them, and even then, they will still probably crack and peel a little bit over the course of a weekend con.
You might be able to find rain boots or galoshes in that batgirl yellow. They won't be as stylish as her boots, but I think they'd work in a pinch.
Unfortunately, boots need to be flexible, while plaster cloth is rigid. So it won't work for this purpose.
Another technique is to make boot-covers. You wear normal shoes, and you sew a thing that looks like a boot, only without any soul to it out of fabric (yellow vinyl fabric if you can find it). You then tack the bottom of the boot cover to the base of your shoes with double stick tape, and maybe a strap or some elastic. You can find details on sewing boot covers with a google search.