There are only a very small number of switch adapted hand-held games machines. On top of this, it seems unlikely that Nintendo's touch screen DS will be ever be made switch accessible due to the technical difficulties involved.
This all said, many standard games consoles can be made portable with the addition of a small LCD screen and power adapter. Of note, there are quite a few one-thumb games available for mobile phones that may be of interest.
Click on the images below for more details on each controller and where you can purchase these from. Please feel free to e-mail for help on anything that is unclear, or to share new information.
The popular Apple iOS range of devices have a confusing array of switch interface which will work with a very limited range of games. Not ideal but giving hope for the future.
From OneSwitch.org.uk (UK)
Adapted SEGA Toys that were originally available with McDonald's Happy Meals. Simple fun games.
From GSTS Designs (USA)
An adapter allowing up to four switches to use a Game Boy Advance. Not all games will be accessible and requires a suitable power adapter. Manual on-line here.