[Radek]

The trident can be found in any "artifact chest" and also as a loot from Demon Lords (Demon Lords can drop artifacts). Once, I got Tonomari Katana in Anephas Shrine as a loot from a Samurai Spirit Guard. This would mean that Samurai Spirit Guards can drop artifacts as well but I haven't seen any other artifact dropped by the guards.
The first artifact chest on your way seems to be the chest with the first tablet in Collasium.

As far as the trident is concerned, it is nothing to write home about. It can't be blessed and it is stoning. If you are longing for a trident then become evil and get the Trident of Posseidon. The Trident of Posseidon cannot be blessed, too.