I used Butterick pattern B5935 as a base for Rapunzel's corset, but you can use practically any other basic corset design. I had to alter the strap enclosures on the pattern I used and add a zipper to the back.
I used this same pattern to make the undershirt, so that they would perfectly align together. I had originally reinforced the seams of the undershirt with ribbing, but it looked too bulky so I removed them. The material I used for the undershirt was actually the same poly-satin fabric that I used for the skirt, only I turned it over to the matte side. The exact color I used is called "Radiant Orchid" and can be found at Joann Fabrics, among other places.