As mentioned in https://www.omnimaga.org/general-calculator-help/where-does-ti-%2884%29-os-store-the-sprite-sheets-for-chars/?topicseen it is different for every OS

What do you want to access the font data for?

There is a way to get access to the font data, though, eg. using the _LoadPattern bcall. You put the character you want in a and the sprite gets loaded to lFont_record (that's an L) if you are using the large font, to sFont_record if you're using the small (_vPutS) font. The first byte is the width of the character, the next seven bytes are the character's sprite. That just loads one character at a time, but you could presumably loop through all the characters and save them into saferam if you really wanted to. The bcall might even set hl to lFont_record for you to make an ldir easier.