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.