One of the sporting goods equipment folks have one listed in their catalog. Rogers, Gilman, or similar. Folks that have sleds etc. Saw one just a while back.
I was the holder, way back in the dark ages, and our kicker just used a shoe string. He would hook a loop over the cleat in front and tie it behind his calf. Never untied it, after he had it where he wanted it, and it was always ready to go.