The fonts are in e.g. ..\Client\data\fonts (Cvet's SDK), other versions should have been using the same path and files.
You will be needing Default.PNG, which is basically a PNG file with one non-flattened layer (i.e. no background). Just open it in PS/GIMP/whatever and see the file properties for yourself.
Be sure to check out the respective FOFNT files too, they contain raw text data for the size and position of each character. For your new characters you may (or may not) need to make slight adjustments there.
Unfortunately there are no specific tutorials about this matter, because Cvet reduced its complexity significantly by converting the fonts into the PNG format. IIRC JovankaB was the only one fiddling with fonts to some extent (fixing edges and form, so that it looks 1:1 with Fallout 1/2, these changes have been accepted into SDK a loooong time ago), maybe she could help you more... Other than that, you can try me again.