Damn little is actually known about Druids, as far as I can tell. The modern adoption of that term by ooo-eee-ooooo wannabes is an ignorant perversion at best. But probably harmless, which beats the crap out of a lot of other religious beliefs.
caw
That's not really fair (calling people "oo ee oo wannabes"). Most serious pagans I've known over the years know that their religion is a reconstruction and not an unbroken tradition or an exact replica of the ancient religions. I realise there are a few that believe, for example, that modern Wicca is a facsimile of pre-Christian Celtic beliefs or whatever, but if they stick around long enough with serious pagans (as opposed to it being just a passing fad they took up for five minutes because it sounded "cool") they get educated and take the whole thing a lot more seriously. Many of them (the serious ones) are very clued up about the actual history of pre-Christian beliefs.
Bear in mind that modern pagans are real people who formed their belief systems for reasons that are a lot deeper than wanting to be like (add any pre-Christian people here). Understand where they're coming from before making judgements.
Back to the OP: "Pagan" is a very vague term which can refer to anything that isn't Islam, Judaism or Christianity (or it's often used to mean any non-mainstream religion, or any pre-Christian religion, etc etc etc), so "pagan tribes" can mean pretty much any group of people who aren't Muslim, Jewish or Christian, anywhere in the world. Probably the reason why you're not pinpointing the info that you need is that you need to be way more specific. Start with one culture. In Europe you have the Vikings, Celts, Saxons, Romans, etc. There are many more than this just in Europe, and each continent has its own huge number of different cultures (more than Europe, as Europe's a very small continent). Druid is a bit more specific - the Gallic/Celtic tribes had druids (remember that the groups I've listed are huge and there will be sub groups within them). I'm not sure if they were the only group that had druids. Around here (southern UK) when people say "druids" we think of the ones that worshipped at Stonehenge. The Celts in England did that before the Saxons came along (i.e. the ancestors of the Welsh, Scottish and Irish before the ancestors of the English displaced them). So now you have a place and time and people, Southern England, Celtic, pre-Saxon. That gives you something more specific to search on, therefore you're more likely to come across historical and archaeological sources. If you can identify an exact date range, even better. Historians and archaeologists tend to classify their info by time and place and the people who lived there.
If you meant druids from some other time and place, then search on that time and place. For pagan, first be specific about which pagans and which time and place. General history books can tell you what people lived where in various regions of the world - even just looking up one specific people (e.g. Vikings) on Wikipedia is a good starting point.