I've got a font converter done to convert any TTF font to the FNT font format used by Rockbox. While any TTF font can be converted, I'd suggest using fonts that are GPL'd or public domain if you wish to have the converted fonts in a theme. Free fonts can be found at http://rbthemes.com/fonts/ttf-fossfonts-0.0.3/fonts/, with additional urls being found in http://rbthemes.com/fonts/ttf-fossfonts-0.0.3/debian/copyright.

As a note, the font sizes are different. When converting to an anti aliased font, the font size entered in the text box will be # pixels. For non-anti aliased conversions, the size will be in points. I don't have a solid conversion, though I believe 12 point font is around 26 pixels on the Sansa. Not quite sure what 12 point font looks like on the Sansa though..

http://rbthemes.com/fonts