PineTab and JingPad seem to be the best bets, but availability is probably a big issue. As is how long they'll be around for, since consumer Linux mobile projects unfortunately have a pretty poor record so far.
Otherwise, you may be able to live with a Chromebook/ChromeOS tablet.
https://www.xda-developers.com/linux-apps-chrome-os/
If you have any old smartphones you're not using any more, you might be interested in PostmarketOS.