all coin location

The 'all' coin cannot be force farmed. Each quest has a chance to drop a couple. As such, you shouldn't really be farming for them. Just hoard them for a while and exchange them at some point when you have a hefty backlog.

Random drop that you can get from any of the free quests. If you have any extra-class Servants (Avenger or Ruler) you'll need it to get their chocolate CE from the shop. Otherwise just exchange them for whatever coin you want/need or exchange them for the chocolates.

I've gotten at least one from every free quest so far (I've done every one available once).