I just learned about this lawsuit which may be why there aren't a lot of accessories available that integrate directly with the phone/case:

https://www.theverge.com/2015/6/1/8696991/blackberry-typo-keyboard-lawsuit-settlement

https://www.amazon.com/Typo2-Keyboard-for-iPhone-6/dp/B00O49D29Y

Interesting review which may apply to most other handhelds:

https://gizmodo.com/typo-2-review-thsi-is-smoe-bulsiht-1676234765