Finding fonts for Digital Publications is not very difficult — mostly you’ll be searching for fonts that are display well on screens.
The more difficult task can be licensing, as every font vendor has different licensing schemes, and some do not even allow you to embed their fonts in a PDF…
When creating native apps or other “encapsulated” file formats, where it is hard for the average user to get to the embedded fonts, I would recommend contacting your font vendor of choice and ask about licensing. Of course you could always rasterize a font, but that’s not really a good choice in regards to accessibility, readability and searchability, especially when using a Digital Publishing solution like App Studio, which by default keeps text as text (HTML5).
But what about ePubs? ePubs are basically a ZIP archive that contain the font file in a reusable format once you unzipped it. And I could understand that font vendors might be hesitant to license fonts for ePubs.
My recommendation: Use an OpenSource font for your reflowable ePub.
As ePubs typically are not displaying as designed, but reflow where line breaks do not matter anyhow, you are probably not so much concerned about CI. Otherwise you shouldn’t really create an ePub, a native app would be the better format.
And remember that in most Readers (hardware and software readers) your customer can switch font, so even if you carefully chose one, they might display everything in Times or Helvetica anyway.
So, if you decide for an OpenSource font that reads well on screen, my recommendation is Droid:
You can find Droid here (and other fonts that are free for commerical use): http://www.fontsquirrel.com/foundry/Google-Android