Rank 1

Rank 1 spells are bought from vendors in Eastern Wastes. In Torment of Velious long duration buffs are no longer ranked, instead are simply sold along with the Rank 1s.

Rank 2

Throughout the expansion trash (rarely), rares (commonly), and chests (always?) drop Minor-Glowing Engraved Velium Rune. Right click on a rune to get a reward window allowing you to choose from any Rank 2 of that level.

Rank 3

Raids in Torment of Velious drop Minor-Glowing Kromrif Etching. Right click on an etching to get a reward window allowing you to choose from any Rank 3 of that level.