There are certain recipes to get Orb of Chance from vendors, known as the Chance Recipes. Below is what you need to trade to the vendor in order to get Orb of Chance from them:
2 x Identified rare items with same name - gives 1 Orb of Chance.
1 x normal, 1 x magic, 1 x rare, 1 x unique with the same base item type - gives 5 Orb of Chance. For example you trade in 1 normal bow, 1 magic bow, 1 rare bow and 1 unique bow to get 5 Orbs.
Full rare set with lowest item level between 1 and 59 - gives 1 Orb of Chance. This is the same as the Chaos Orb recipe, but with items pre-level 60. If all items are level 60+ except for one, you get an Orb of Chance instead of Chaos.
Full rare set with lowest item level between 1 and 59, all unidentified or all 20% quality - gives 2 Orb of Chance. Again, same as the 2 Chaos Recipe. Usually not really worth it.
Full rare set with lowest item level between 1 and 59, all unidentified and all 20% quality - gives 1 Orb of Chance. Definitely not worth the currency you need to spend to get all 9/10 items to 20 % quality.
Along with the vendor recipes, getting Orb of Chances is best to trade from other players or get from drops. You can usually get 5-10 Orb of Chances for a single Chaos Orb by using poe.trade.