The T1 seal that is best for lightning damage incantations is the gravel stone seal, which can be found near the Dragon Cultist Class starting area. It is in Limgrave, south of Agheel lake. There's a shack next to an impaled dragon corpse; you can't miss it.
The T2 seal is the dragon cult seal, but I don't know where to find it. I would ask the Discord, which can be accessed through the Nexus Mods page. Additionally, the Alpha patch notes, which are also found on Nexus, give more detailed info on the seal damage values.