I used to make my own as a kid. (Find a piece of black card. Smear a thin layer of lard over it. Place greaseproof paper over the top and use a blunt pencil, or similar, to write or draw with. Lift paper to clear).
If you're looking for them online though, it might help to know that they're also called magnetic drawing boards. e.g.
https://tinyurl.com/yx6mabcx
These days you can get an electronic version too. (Search for 'LCD writing tablet'). e.g.
Amazon.co.uk User Recommendation