The font files are located in the ARC file Fonts.arc (Titan Quest/Resources/). There are three files with the extension .fnt:
- Albertus MT Light.fnt
- Albertus MT.fnt
They are just renamed truetype fonts.
Use ARC Explorer to extract them.
To use your own fonts in game, rename the desired font files to these names, and create a new Fonts.arc file with the Archivetool utility and then backup your original Fonts.arc file before replacing it with the new one.