Path of Exile 2 is a next generation Action RPG created by Grinding Gear Games. Set years after the original Path of Exile, you will return to the dark world of Wraeclast and seek to end the corruption that is spreading. Path of Exile 2 is a free-to-play online multiplayer game with co-op for up to six players. 
I’m trying to collect a list of the best spirit sources for stacking to estimate the maximum possible spirit.
Questions, comments, and additions welcome!
Act 1, The King in the Mists: Gemrot Skull, +30 to Maximum Spirit
https://www.twitch.tv/videos/2319800982?t=06h44m13s
Act 3, Ignagduk, The Bog Witch: Gembloom Skull, +30 to Maximum Spirit
https://www.twitch.tv/videos/2320709487?t=03h19m46s
Act 6: Gemcrust Skull +40 - Ignagduk, The Bog Witch: Gemcrust Skull, +40 to Maximum Spirit
https://www.twitch.tv/videos/2320709487?t=16h27m30s
Beidat's Will, +1 to Maximum Spirit per 25 Maximum Life
Alpha's Howl Armoured Cap, +100 to Spirit and +30 to Spirit (Enchant)
Oaksworn Sigil Crest Shield, +50 to Spirit
Enfolding Dawn Pilgrim Vestments, +100 to Spirit
Ventor's Gamble Gold Ring, +(0–20) to Spirit
Amulet, +50 to Spirit
Scepter, +120 to Spirit and (61—65)% + (31—34)% increased Spirit (92—99)%
Grand Spectrum Emerald, 2% increased Spirit per socketed Grand Spectrum