This entry on Illithid Powers mentions the locations of 41 tadpoles. I'll copy-paste (and edit) the locations from there, add the ones from this video on YouTube mentioning 2 more, and any others I come across:
Obviously, spoilers ahead!
Below you can find an overview of the Illithid Powers menu.
Since an official source on the amount does not (yet) exist, I'll try to keep this updated.
ACT I, 8 tadpoles:
- One can be found on the dead True Soul in Forest (X139, Y444), do not resist the tadpole to acquire.
- One can be found by defeating Flind, the Gnoll Warlord (X33, Y594) (Quest 'Find the Missing Shipment').
- One can be found in the Druid Grove (X445, Y-17), it would be on a table in Halsin's/Nettie's Secret Research Lab next to the Dissected Drow body.
- One can be found by defeating True Soul Gut (X297, Y-12).
- One can be found by defeating Nightwarden Minthara.
- One can be found by defeating Dror Ragzlin.
- One can be found by defeating True Soul Nere.
- One can also be given to you by saving Nere, and keeping him from dying during the fight with the Duergar. If you fight and kill him afterwards you can pick up a second one.
- Note: if you side with the Absolute, you can get as many Illithid Powers as "Gift by the Absolute" as you missed by not killing the True Soul bosses.
ACT II, 28 tadpoles:
- One can be found by defeating Fist Marcus.
- Three can be found at Infirmary, Creche Y'llek.
- One can be found on Linsella, Moonrise Towers Kitchen.
- One on a body in Oubliette (X:628, Y: -92).
- Three at the front entrance of Moonrise Towers from Disciple Z'Rell, Zealot Malik, and Adept Merim. The linked video explains how you can take out Z'Rell at an earlier point.
- Two in the barrel on the docks of Moonrise Towers.
- One in the Brine Pool in the Tadpoling Centre in the Mindflayer Colony.
- The Astral-Touched Tadpole, given to you by the Dream Visitor/Emperor, immediately develops certain Illithid skills, having the effect of up to 15 tadpoles, depending on what skills you have unlocked (in addition to the Tier 1 powers, it grants you Absorb Intellect, Freecast, Mind Sanctuary, Mind Blast, Black Hole, Psionic Dominance, Illithid Expertise, Fracture Psyche, Displacer Beast Shape, and Fly).
Any tadpoles used for unlocking Tier 1 powers will be reimbursed.*
ACT III, 23 tadpoles:
- A newborn Mind Flayer can be found in the Abandoned Windmill basement at Rivington.
- One on a beach northwest of Rivington, on a barrel next to the Stone Lord thugs about to fight the Guild members (X: -223, Y: 105).
- A further two on the ship the Stone Lord thugs arrived in, in the big 'Storage' unit. (X: -211, Y: 135).
- One from killing Avery Sonshal at Felogyr's Fireworks, Baldur's Gate Lower City.
- Two can be found on Enver Gortash's Parents, the Flymms (X-30, Y-116), Baldur's Gate Lower City.
- One can be found in High Security Safe n6 (near X-715, Y882) in the high-security vault of the Counting House.
- One can be found sitting on a table on the ground floor of Sorcerous Sundries (X-8, Y-91).
- One can be found on the second floor of The Lodge, Lower City (X-207, Y-87).
- One on a table in the Sanguine Laboratory, entered from Baldur's Gate's Lower City down a hatch (X-92, Y-91).
- Six can be found on a ship protected by Steel Watchers, in front of the Foundry.
- One additional one can be found on the body of Churg Elvek, who is preaching next to this ship.
- One can be found in Iron Throne, on the table near Omeluum (x-45, Y17).
- One can be found on a desk in the Steel Watch Foundry - Lab Level (X-1940, Y447).
- One can be acquired from Manip Edenosa in Bloomridge Park.
- One can be found in the basement of the Blushing Mermaid.
- The Emperor gives you another special tadpole, potentially unlocking all skills if one of your troupe is willing to evolve completely (so it basically counts for 25 tadpoles, hence the extra number in the grand total).
Grand Total: 59 (83) tadpoles
So you can unlock all powers of three party members (3x25) around the end of the game and still have a few tadpoles left.