They are made in the UK by a company called Bloccs. Quite expensive, but they are good quality. My original one is a small child's arm length one. I have just ordered a larger one, for an older child and a full length adult leg one, as per the enclosed picture.
Latex stockings seem to be of poor quality these days, so I am hoping the adult leg cover is going to be a suitable alternative. The original arm cover was a lovely ride and I'm hoping that more of us will try them and post stuff to Youtube... It would be awesome to see someone do a BTP with one