There's a chapter in Tacitus' Germania which goes like: "the Cotini, more to their shame, also mine iron." (Germania, 43, translated my Harold Mattingly) The explanatory notes says that the "more to their shame" comes from a Roman perspective - slaves did the mining.
Surely one tribe cannot be responsible for the entirety of the iron produced in Germania? On the other hand, from what else Tacitus writes, I cannot imagine the Germans having the centralisation to mine iron in vast quantities that was needed. I also cannot believe that the Romans would supply the Germans with iron and arms - a group of people that threatened their way of life.
As an extension, how would, say, the Gaulish, Brittonic and steppe tribes get the iron that they needed? I can imagine the Gauls and Britons having a stronger central government than the Germans that was able to co-ordinate mining (since they had more elaborate power structures) but not the nomads.
One source is bog iron.
This page provides a pretty good explanation of it in the viking era but it was around long before that and was in use in pretty much every place that had peat marshes or bogs.
My understanding, from reading Braudel, is that easily accessible iron deposits were widely spread (in contrast to tin for making bronze).
Bear in mind that it must have been a lot easier to find surface deposits in ancient times, before they had been worked out. I think you can see a sort of shallow pit topography from small-scale mining in the hills not far from where I live.
As a follow up question and possibly a question in aiding your own given question.
Who did the European Barbarians usually trade with? I say usually to represent people they trusted/allied or controlled.