The key to unlocking new strains is by breeding Epic Buds together in the Breeding Lab! Once your strain reaches Mastery level 2, there is a chance for an Epic Bud to drop from any harvest of 10 or more buds from a single plant. With the exception of a few cases at the start of the game, all new strains come from breeding the right combination of Epic Buds together in the Breeding Lab, so try out a variety of different match-ups, and keep in mind that a particular combination can have different results if you switch their mother and father slot roles! Keep growin' those buds and try out everything you can think of!
For more information check the Breeding Lab section of the FAQ! If you're particularly stumped trying to discover a strain, then try asking other players by posting on our forum in the Cafe building, or just keep leveling up your Hempire, as each strain is automatically unlocked once you reach a certain level!