Where find Pure Silver
Oh good you didn't progress too far then.
Once you complete the Iselia ranch event, do the events in Triet and Triet Ruins.
Once you reach the town of Izood (coastal town) save and head back to Hakonesia Peak. There will be a sidequest you can do there with an old friend (or enemy depending on your view).
Doing this sidequest will leave you without Marta and other human allies during the sidequest but it will reopen the Asgard Stone Dais dungeon and a new area hidden within it.
After he points out a few clues, you can open the door that was locked near the entrance to access the Balacruf Mausoleum, and just outside it you'll see a Goliath.
Be careful though, the enemies here are around 20-30 in lvl so don't get careless.