Sword Coast Adventurer's Guide

What do I need to buy to get access to Feral Tiefling?

I don't think they are available without the Sword Coast Adventurers Guide. Mordenkainen's Tome of Foes adds a lot of tiefling bloodlines, but not that particular one. The changes are very minor however, and you should be able to make manual edits.

Thank you, I couldn't find SCAG on roll20 so I figure I would have to do that.

Unfortunately it is the only source book no currently available for fifth edition. It is extremely frustrating for me personally as it has so many spells and other cool races that are a pain in the butt to add in manually every time my players want them. I mean seriously it has been almost 4 years since it was released! I am a bit sore about Out of the Abyss too.