The Gauls believe the power of the immortal gods can be appeased only if one human life is exchanged for another, and they have sacrifices of this kind regularly established by the community. Some of them have enormous images made of wickerwork, the limbs of which they fill with living men; these are set on fire and the men perish, enveloped in the flames. (Caesar de bello Gallico VI, 16)

But what many don't realize, Caesar isn’t our only source.

We are told of still other kinds of sacrifices....having built a colossus of straw and wood, they throw into the colossus cattle and animals of all sorts and human beings, and then make a burnt offering of the whole thing. Strabo Geographica IV, 4, 5

Most scholars agree that Poseidonios (c. 135-50 BC), who travelled widely in Gaul, is our oldest source and that Strabo and Caesar who are almost contemporaries probably got part, if not most, of their information from him.

...and those Gauls who propitiate with human sacrifices the merciless gods Teutas, Esus and Taranis - at whose altars the visitant shudders because they are as awe-inspiring as those of Scythian Diana. Lucan, Pharsalia 1 422-465

..... It is a custom of the Gauls that no one performs a sacrifice without the assistance of a philosopher, for they say that offerings to the gods ought only to be made through the mediation of these men, who are learned in the divine nature and, so to speak, familiar with it, and it is through their agency that the blessings of the gods should properly be sought. Diodorus Siculus Histories V.31

Although there can be no unequivocal evidence of a wickerman, there is evidence of large scale ritual use of fire. In October 1997, archaeologists working in the Austrian town of Leonding excavated a deep pit, dating to about 200 BC, containing the remains of approximately 12 humans and animals which appeared to have been subjected to burning. The site has been interpreted as the focus of a ritual where humans and animals were tethered to some sort of wooden device and suspended above a pit which was then set on fire so that after a while, everything collapsed into the pit. (Dying for the Gods, Dr. Miranda Green.)

Sources

Green, Miranda Dying for the Gods Tempus Publishing Company, 2002

Green, Miranda, The World of the Druids Thames and Hudson, 1997