If you're converting to epub using Calibre, see post #10.
If not, you need to:
- Open the epub with an unzip utility (e.g. 7zip, WinRAR, Calibre Tweak epub)
- Find the .css file and edit it in your favourite text editor. Copy all the @font-face statements to the top of the file (but after the css header, if there is one)
- Replace the .css file in the epub and you're done.