Shark's Blood can be found in Golden Giftboxes, which means it should be one of the more common components (compared to the ones found in Philosopher's Caches.) The reason it's impossible to find on the Marketplace is twofold: not enough people want to go to the trouble of creating Golden Giftboxes, and Shark's Blood is useful enough at low levels that it tends to get bought within a few seconds of being listed.