There isn't a spot on HR FF. There's only one spot in Tundra, it's area 4 I believe. It's the top most area. One of the mining nodes there has it. I think it's the node closest to the entrance to area 3.
It's also in HR Volcano, but I don't know where.
And TC, just so you know, you don't need that ore for Volvidon S armor. I made that armor too. Volvi S feet and regular S feet is Torso up, so you don't need the ore for the S armor. Just use the low rank one.
In Area 4 of the Tundra in high rank, one of the mining spots has a 7% chance of giving Fucium Ore. The guide doesn't specify where the exact mining spot is located, but I think it's the one next to the entrance to Area 2. I could be wrong though. :/