Gold upgrade boost can be obtained from all bosses at diamond level (except for Gold Goblins)
An Upgrade Boost is a card that can be directly converted into dust for the purposes of upgrading a card. These cards come in four varieties, shown left. Each one has different methods for obtaining it. You can get them by events and by buying them with gems in Packs or Boxes. Also, some bosses drop them as a prize in the prize pool. Bronze, Silver and Gold Upgrade Boosts can also be obtained with Dark Orbs from Arena, although the Silver and Gold Boosts require a certain rank to purchase. You can buy 4 bronze boosters for 1200 gold, however it is not recommended to do so, since you can buy 6 boxes of cards, where even if all the cards are bronze, you will get the same amount of dust, and even have a chance of getting other rarities of cards.