What is the fastest/easiest way to get Forgotten Souls?
Right now I have quite a few legendary materials, but not a single Forgotten Soul, so I can't craft any of these level 70 legendaries.
I've found that doing rifts with a good party is the fastest way. The rift guardian has a pretty high chance of dropping a soul and legendary drops are boosted while in rifts. There is no other good way really of getting souls other than rifts.
As of the latest patch (2.1) you can no longer do the following as everyone needs to contribute a key to enter the rift:
An even faster way now is to join the Rift it Forward community (assuming you have enough keystones to open rifts) to quickly farm Rift Guardians as well as to gain blood shards for gambling purposes.
With this community, you can gain hundreds of souls very quickly.
There are two ways for getting Forgotten Souls :
I usually clear T1 rift and salvage every legendaries that is not usefull for my character. I currently have more than 200 unused Forgotten Souls. Plus the bloodshard you get from the rift guardian allow you to gamble at Kadala significantly increasing your legendary "drop" rate.