Run dependent, but very attainable early on. The well of Charon are the purple fountains that spawn in rooms randomly throughout your run.
Using coins here will give you all sorts of benefits, including buffs, darkness/gems, spawning fish/troves, and healing/getting back death defiance’s.
Obviously the cheaper items are gonna be the way to go here, and if you don’t spend any money in normal shops and buy 2 items every well you run into, you should hit the 9 required.
Some boons to look out for to help are:
-Sunken treasure (Poseidon): gives a random assortment of loot items (health, coins, darkness)
-oceans bounty (Poseidon): gives a bonus % to loot items found that run
-side hustle (Hermes): great boon in general especially if you can get it near the beginning of a run, it gives a flat amount of coins every time you enter a new room
-any chaos boons that increase the % of loot items/coins that run
Besides that you do have to get lucky with the amount of wells you run into in a run. Personally I got it by just buying the health items and the extra gems/darkness, as those seem to be the cheapest.