Have you tried Wacom?

They have their own TablePCs and manufacturers make Tablet PCs with their digitizers.

There is a community made linux drivers for them, compatible TabletPCs are listed here: github.com/linuxwacom/input-wacom/…/Device-IDs#ta…

They also make touch screen monitors that start around $300 (up to thousands) that connect to any PC and should also work under Linux, afaik. Or you could get the cheap ‘old school’ tablets made from plastic that simply move and control the cursor, though they take a while to get used to, IMO.