You could do a google search and just grab the first font.h that you find but there's no guarantee that's the one you need. It's likely to be a common filename.
The key is to get some sort of context to indicate what this file is and use that information to guide your search. It's not as if the need for this file was just mysteriously dropped on you by the gods. Somewhere you found some code that requires this file, don't you think wherever you found that code would be the place to start? Since you haven't provided any information there's nothing I can do to help you further.