Where can I get a Red Soul Shard?

From the Red Soul Shard wikia page, it looks like this gem is only dropped by the Dark Lord himself as part of the yearly Darkening of Tristram event (note: this event is new as of 2017):

It can only be socketed into helms, and is obtained by completing the Darkening of Tristram event in January of each year. It drops from Dark Lord (Diablo).

From the event page for the Darkening of Tristram, it looks like this is a guaranteed drop for completing the event dungeon:

Completing the Dungeon also rewards a Legendary Gem, Red Soul Shard, that fits into the Helmet slot, reminiscent of Diablo's Soulstone (into which it will transmog the helmet).