How to get Fragmented Souls
Here's a good guide on how to get fragmented souls to buy Festival of the Lost masks. The easiest way is to get bounties from Amanda Holliday, usually you get just one it seems but other bounties can have higher fragmented souls rewards like +10. Daily bounties give you +1, Weekly bounties give you +40 fragmented souls. That's enough to buy a Festival of the Lost mask!
How to get all the Festival of the Lost Masks
Forbes has the full list of all Festival of the Lost masks. The first mask you get is Master Rahool. Each mask can be upgraded by defeating nightmares (the bosses at the end of each "branch" in the Haunted Forest quest, usually you defeat 10 to upgrade the mask's perk).