My Ursula costume is completely handmade--even the shell necklace.
I created my own blubber from foam and designed the skirt around it. The foam was 1 inch thick and rolled into a tube. I made a skirt form with quilting batting and attached the foam tube on the inside.
The necklace is made of simple modeling clay painted in gold metalics. I rolled up a few inches of tinfoil to bulk out the thickest parts. This allowed for more even baking in the oven. I used super glue on the seams to ensure that the coil stuck together.
The corset top is short and stops at the waist. I used fabric covered boning and fusable interfacing for structure. The front has hook and eye closures. The back has grommets and braided leather cord to tie it together. I couldn't find black grommets for some reason, so I spray painted them.
The legs are fully posable! They're filled with the same foam and quilting batting as the blubber. I placed a tube of the larger foam over another piece of foam that was the same width as the tentacle. Then, I surrounded it with the thinner batting to create a smooth shape. I tried using all sorts of wires braided together to pose the tentacles, but the only thing that allowed for easy manipulation was actually two wire hangers running down each side of the tentacles. The wire hangers hold shapes while still being easy to bend by hand.
The tentacle suckers are suction cups painted and sewn onto the fabric.
Underneath, I added a layer of sheer blue material to simulate the ocean (and hide the legs).