Discovery

Are you referring to exotics? If so, you'll have to buy them from Xur on the weekends. If you buy a "legacy" engram, which is a random year 1 exotic of that type, you'll then "discover" it and have the option to buy the year 2 version of it through the kiosk in the hall of guardians in the Tower. Edit: You can also buy 3 of coins from Xur by using your strange coins.. Using one before an ultra boss, will increase your chances of an exotic engram to fall. If one doesn't fall after the kill, your chances of one falling from the next ultra boss will increase with your next 3 of coins used. It's always a good idea to spread out the usage of 3oC for 15-30 minutes IMO as there's a "cool down" period for them. I usually get an exotic with using 3oC with a 1 outta 5 used.