Reiterating what Raz415 said - at some point, picking up tadpoles no longer drops a jar in your inventory, but rather, immediately increments the number of tadpoles available up by one.
Here are all tadpole locations.
TADPOLES IN ACT 1
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 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.
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.
TADPOLES IN ACT 2
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 (Z'Rell may be inside instead of outside at the foot of the stairs.)
Two in the talking barrel on the docks of Moonrise Towers.
One in the Brine Pool in the Tadpoling Centre in the Mindflayer Colony.
One special (astral-touched) tadpole can be acquired after you helped the Dream Visitor before reaching Baldur's gate.
TADPOLES IN ACT 3
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, see Rivington map)
A further two on the ship the Stone Lord thugs arrived in, in the big 'Storage' unit. (X: -211, Y: 135 see Rivington map)
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.