I've converted, for my own use, a lot of third-party fonts to rockbox format using the convttf utility (the anti-aliasing one). Most of these came from free font sites like:
https://www.dafont.com/The problem is I don't really understand the licencing issues, and what can or can't be distributed (e.g as part of a theme). There seems to be a _huge_ variety of different licences fonts are released under, ("public domain" "apache licence" GPL, CCA, and including some that just say things like "free" or "do what you want with it").
Many of the good ones seem to be under the SIL 'open font licence'.
http://scripts.sil.org/OFLBut I can't work out if that is compatible with the terms for rockbox. It's not the same licence as Creative Commons, but is it compatible with it?
Feels like you need to be a lawyer to understand this stuff.