I've seen them at Kmart and Bed Bath and Beyond.
Yesterday at BB&B, a customer was trying to find two that worked. They had the child try them and were trying to squeeze them to test the mechanism. Out of about 10 pairs that I saw, only 1 slipper worked. I was watching because DD desperately wants these, but based on that and the 1/2 priced returned ones that didn't work, I'm thinking we'll pass.