Ronald Hutton in Blood & Mistletoe: The History of the Druids in Britain (2009) actually does a really great job discussing this, in particular in the first chapter titled "Raw Material." The gist is that there are very few sources on the druids or ancient Celtic/Gaelic religion in general, and the only roughly contemporary sources are from Roman and Greek authors, the most famous of which being Julius Caesar's De bello gallico (The Gallic Wars), but also Marcus Tullius Cicero's note on Druidic divination, Diodorus Siculus, Strabo, and Pomponius Mela - and that is pretty much all of the contemporary material, and even then we are left with a very small core of somewhat conflicting data, a good chunk of which may or may not be derived from a lost work by the Greek authority Posidonius.

Post-Roman conquest of Gaul, Roman sources on druids show heavy influence from Caesar & co.'s preconceptions, and more political angles begin to influence the writing, as Emperor Claudius banned and suppressed the religion in 40 C.E. and it is difficult to sort out the political and poetic rhetoric from genuine reporting in the writings Suentius, Pliny, Marcus Annaeus Lucanus (Lucan), and Tacitus - and the degree to which any of them are reliable reporters is another issue entirely. There are later accounts in the 1st and 2nd century CE, but as Hutton says (19-20):

Put together, this group of sources is vulnerable to disputation in both general and particular respects. The general problem with them is that they seem to be written mostly by people who were even more removed in geography and time from the societies that had produced the Druids than the other authors who dealt with them, and even more dependent on second- and third-hand testimony. they are certainly briefer and less detailed than the descriptions given in the more hostile accounts, and can be dismissed as the work of intellectuals constructing a myth of noble savages with which to criticize their own civilization.

Of written accounts from Druids themselves, there are none. Archaeological remains are often subject to interpretation and misinterpretation. The best example of both is the artifact known as the Coligny calendar, a fragmentary bronze object marked with a Gaulish language in Latin characters, dated to the later second century CE discovered in France in 1897. The fragmentary nature of the find makes interpretation difficult. It has been described as a "druidical device" by writers like Miranda Aldhouse-Green, but there is actually no contextual evidence to link it to the druids.

So beyond Classical sources and archaeology, the major source of references were in medieval Welsh and Irish literature - much of which was written hundreds of years after the Druids had ceased to be a living tradition. The Celtic Twilight or Renaissance in the late 19th/early 20th century in particular dovetailed with a renewed interest in Welsh, Irish, Scottish, Cornish, Manx, etc. history, folklore, and literature, including popular translations and publication of such material into English.

It should be understood at this point that there were several different movements in occult, literary, and scientific circles which had varying degrees of overlap and association with druidry. The 18th-19th century saw the rise of fraternal organizations associated with or based on Freemasonry, these included explicitly non-occult groups which took on the popular image of druids such as the Ancient Order of the Druids (AOO), and explicitly occult groups such as the Hermetic Order of the Golden Dawn and its offshoot the Ordo Templi Orientalis (OTO). The Golden Dawn in particular had crossover membership with members of the Celtic Twilight literary revival such as William Butler Yeats and Arthur Machen.

Druidry, particularly the Welsh form of it, caught the popular imagination thanks to Iolo Morganwg, a Welsh scholar and poet who claimed connection to (and in fact to be the last living representative of) the medieval Welsh bardic tradition. Iolo managed to successfully forge and publish a number of influential writings supposedly translated from authentic Welsh medieval manuscripts, and in the process his conception of medieval Welsh bards - and their connections with druids - were muddled together, and were largely accepted as genuine. This led (in the 18th century) to a "revival" of the bardic gorsedd and the foundation of an order of Bards, Ovates, and Druids (the ranking based on ancient Roman sources); Iolo's theatrics caused a response, most notably Davies' The Mythology and Rites of the British Druids (1809), which would in turn further popularize much of the mythology of druids that had been built up during the last couple hundred years.

Anthropology and archaeology were in their infancy when the first association was made between various earthworks and monuments in the British Isles with druids, most notably in William Stukeley's Stonehenge: A Temple Restor'd to the British Druids (1740) and Abury: A Temple of the British Druids (1743). These early associations of ancient monuments with the Druids, as well as reliance on Roman sources and fictional material from Iolo, strongly impacted serious archaeological and anthropological work around druids, Stonehenge, etc. well into the 20th century, so when Margaret Murray published The Witch-Cult in Western Europe (1921), she wrote:

Whether the religion which survived as the witch cult was the same as the religion of the Druids, or whether it belonged to a still earlier stratum, is not clear. Though the descriptions of classical authors are rather too vague and scanty to settle such a point, sufficient remains to show that a fertility cult did once exist in these islands, akin to similar cults in the ancient world. Such rites would not be suppressed by the tribes who entered Great Britain after the withdrawal of the Romans; a continuance of the cult may therefore be expected among the people whom the Christian missionaries laboured to convert.

Which brings us down to about the neopagan and particularly Wicca influences. In the early 20th century there was a large body of "druidic" material which varied from Classical Latin sources to the Welsh bardic material (some of which was fanciful) to archaeological and anthropological material (some of which was heavily influenced by the above, and just plain wrong), and then purely occult/religious material - some of which didn't have much to do with ancient Druidry at all. There were fraternal groups calling themselves druids and religious groups claiming to be druids and the Welsh revival of the gorsedd which retained some elements of Iolo's druidic devices.

So there was a lot to choose form...and Gerald Gardner, who essentially created contemporary Wicca with the publication of Witchcraft Today (1954) borrowed from all of it really - Murray, the Golden Dawn, popular conceptions of Druids, etc. You can see the muddle when he writes (44):

This may derive from the time of the associations of Druidesses of whom the Romans spoke as witches. Whether these were true Druidesses I do not know. It seems to have been a separate religious organization, possibly under the rule of the chief Druid, much in the same way that there was a priest or someone who might turn up at a witches' meeting and be acknowledged chief who came to be called "The Devil" in mediaeval times. I think the use of the witches' circle, in magic, may have come from the Druid, or rather the pre-Druid, people, who built Stonehenge and Avebury and who made use of it to concentrate the powers generated. It is a direct descendent of the circles used in the prehistoric cave magic, though of course it may have come from the East. The Romans suppressed the Druids in the areas they effective occupied, but I think it possible that a women's section may have carried on even there, perhaps in secret [...]

So to answer your question: there were a lot of sources, but very little of the information was accurate or reliable, though not everyone would have been able to distinguish between fact and fiction.