They are made in Britain so that's good news, for us in Britain that is :o) The address is.... Lenric c21 Ltd, Lineside Industrial Estate, Littlehampton, BN17 7LU. They have a web site www.snugglesafe.com
You can use them with or without the cover, they claim to hold the heat up to 12 hours. Sue has used them regularly in her pens and she says they are great for keeping the kitties warm in the winter. I suppose they would be just as effective if you put them in your own bed. A hot water bottle is a cheaper alternative, but my bottles cool down after a couple of hours, but by that time I am fast asleep.
In the kitties room I have a garden chair with a cushion on it. I put one under the cushion and sat on it while I was playing with them. I could feel the warmth come up through the cushion. Perhaps it's worth looking in pet stores, such as Pets at Home, or good quality Garden Centres where they have a pet section.
I hope this helps, good luck in your search.
PS. I have just checked out the Snugglesafe web site. It's good for additional information, but they are far too expensive to buy direct from them. Look elsewhere.