Are there any primary sources regarding Druids, especially related to Emperor Claudius banning Druidism?

There is, indeed, primary source material on Emperor Claudius and the Druids. The problem, however, lies in the perennial difficulty surrounding ancient texts, the transliterations and translations of which may conflict and may not always be reliable. Another caveat, however, is the fundamental question of what constitutes "primary source material" when discussing ancient texts, such those written by Tacitus, Herodotus, Suetonius and others. Often, such highly-esteemed figures in the study of history are drawing from...

There is, indeed, primary source material on Emperor Claudius and the Druids. The problem, however, lies in the perennial difficulty surrounding ancient texts, the transliterations and translations of which may conflict and may not always be reliable. Another caveat, however, is the fundamental question of what constitutes "primary source material" when discussing ancient texts, such those written by Tacitus, Herodotus, Suetonius and others. Often, such highly-esteemed figures in the study of history are drawing from questionable sources, yet these early historians are usually the only sources of information we have on the periods they cover. Suetonius, for example, remains widely studied for his portraits of the leaders of ancient Rome, yet how reliable is Suetonius on such a vast topic with the obvious limitations under which he was studying and writing. That said, this link provides about as good a reference as one is likely to find on the topic of Claudius and the Druids. While the passages provided are, by necessity, abbreviated, they do provide an optimal guide for further research.

Another very good source is the volume edited by John T. Koch, titled Celtic Culture: A Historical Encyclopedia, a multi-volume history that draws on pretty much the same sources as those to which the above link references. You won't be able to purchase this set of encyclopedias unless you are willing to spend hundreds of dollars, but you should be able to locate them at a university library.