Wherever you found them,does it say what they are made of?? Would pvc work?/ I don't have a clue what these look like,maybe if you post a picture of the ones you found,we'd be better able to help you figure it out.
Kathi

Maybe something like this:

Here is a link that might be useful: Heat moldable stabilizer

You can use "pool noodles" - those floating plastic tube things - and duct tape.

Cut it as long as the height of an average boot plus 5-6 inches. Cut a notch out of it where the "ankle" should be and bend the thing in to a foot shape. Tape it into position and slide a really pretty knee-height sock onto it.

Tah-Dah!

Here where I live you can get pool noodles at the .99 cent stores.
Kathi