You can get those at ebay, also this site has a good variety;
https://germanmanicuresets.com.au/hot-water-bottles.html?gclid=EAIaIQobChMI_bOAkPPP8wIVBzErCh2kfgVUEAAYASAAEgL6XvD_BwE

I second to be careful with burning. I always do first a large cup size of cold water in the warm water bottle, then add hot.

I work in a hospital and see many burns caused by hot water bottles, so please be careful people!

Chemist’s usually have good hot water bottles!

My mum buys hers from Ebay.

Thanks – will look at this place for a hot water bottle cover.

We have a shop called Opal World that specialises in sheep skin products. Not sure if they have hot water bottle covers though. You could buy scrap sheep skin and put together your own???