The ever-useful site l'arbre celtique lists a few classical references to Taranis - the 'Civil War' (or 'Pharsalia') by Lucan, plus sholarly glossess on Lucan's text. The translations there are in French; English translations are available here. The main reference is book 1, 441-446 (unfortunately this English translation is in a very nineteenth century heroic verse):

Thou, too, oh Treves,
Rejoicest that the war has left thy bounds.
Ligurian tribes, now shorn, in ancient days
First of the long-haired nations, on whose necks
Once flowed the auburn locks in pride supreme;
And those who pacify with blood accursed
Savage Teutates, Hesus' horrid shrines,
And Taranis' altars cruel as were those
Loved by Diana, goddess of the north;
All these now rest in peace. And you, ye Bards,
Whose martial lays send down to distant times
The fame of valorous deeds in battle done,
Pour forth in safety more abundant song.