Draconic Vis Wax

The wizards of the Wizards' Tower have managed to artificially generate vis wax, through the Rune Goldberg Machine. It takes piles upon piles of runes and distills them to a thin, waxy substance. It's possibly the least efficient process in the world - the equivalent of sieving a mountain to get a couple of nuggets of gold.

The alternative is to find vis wax in its natural, unfiltered state, but that has its own mountain-sized problems. You need to siphon it from the most dangerous of dragons, and those dragons need to have been in a state of intense rage when they died. It's enough to make a wizard sigh and approach the mountain with a newly bought sieve.