I have seen it both ways:
He is a veritable font of information.
He is a veritable fount of information.
The first is referenced by M-W's definition and seems to match the pronunciation I'm used to:
source, fountain (a font of information)
The second seems equally sensible given that 'fount' can be an abbreviation for 'fountain'.
An NGram shows that 'fount' outpaces 'font' in written usage (seemingly in contradiction with the dictionary), but both seem to be pretty widespread.
Are there any reasons to use one over the other, or are they really wholly interchangeable?