Many tablets can run GNU/Linux distributions.
Some big laptops can also be used as tablets. Once modified, these can boot with free software and some might even have the RYF certification, depending on availability by vendors:
- Thinkpad X60 Tablet
- Thinkpad X200 Tablet
Some ARM tablets can also probably boot free software, but they would also need to be supported by FSDG compliant distributions. If the tablet is well supported in Linux and u-boot, that is usually easy to do if the distribution supports ARM. See the single-board-computers article about more information on the system on a chip that can boot with free software. And the Group:Hardware/FSDG_distributions article has some information about ARM support of FSDG compliant distributions.