Where to purchase Coldain Velium Temper
Coldain Velium Tempers can be purchased in Thurgadin, the town of Ice dwarves located in northern Great Divide. You can reach Thurgadin by taking the Great Divide PoK book or by asking a Druid/Wizard to port you to The Great Divide. If these methods aren't available to you, you'll have to make the long hike here from Iceclad Ocean.
The NPC you're looking for in the town of Thurgadin is Nimren Stonecutter. He's in the northern portion of the town, shown on my map below.