Here is Open Sans, I even corrected it for Kobo Kepub and eInk. Just put it to /fonts folder.
http://drop.muratcansimsek.com/OpenSans.zip
I don't know why anyone would ever want to use an UI typeface for text reading, but here it is. But there are much better sans-serif typefaces for reading, like Kobo's own Gill Sans and Avenir.