The Destiny Rod is one of the best Rods in Fisch, but you won't find it at any merchant in the game. You can find it in an unmarked spot in the ocean, but you'll need to complete an achievement before you get the chance to buy it.

You can find the Destiny Rod with Caleia in an unmarked spot called the Arch. This is located near Moosewood, where you first spawn in Fisch and Mushgrove Swamp. Grab your boat and go right as soon as you leave Moosewood and head towards Mushgrove Swamp. Before you arrive, you will see a stone arch to the left of Mushgrove Swamp.

I ended up missing the Arch and reached Mushgrove Swamp first because it wasn't rendering fast enough to see. If you're heading towards Mushgrove Swamp and don't see the Arch, back off a bit and sail slowly to the left so it can render.

When you arrive at the Arch, you’ll see a character dressed in blue sitting on the left side of the Arch’s base, with their feet in the water. Their name is Caleia, and they’ll ask to see your Bestiary. Show them everything you’ve collected so far, and they’ll tell you that you need to discover 70 percent of all the fish in the game. You can purchase the Destiny Rod for C$190,000 once you’ve completed her request.