Tacitus published his Germania in 98 CE. He intended to describe the land and its inhabitants to the Roman ruling class. Posidonius the last of the great Greek historians, was the first (c. 135-51 BCE) to give some account of the Germans. Tacitus took some of his information from the Histories of Posidonius. He had no other Greek source. Strabo's work was unknown in the Roman west.
The books of Caesar and Livy were available to Tacitus, in particular book 104 of Livy's history describing Germany (lost to us). Where Tacitus and Caesar cover the same ground, the former is usually more detailed and up to date. The elder Pliny did military service in both Upper and Lower Germany and his Bella Germaniae will have been used by Tacitus, but to what extent we cannot tell due to the loss of this work.
During the twenty years separating the work of Pliny from the publication of the Germania, Tacitus will have collected the newest information from soldiers and traders. Roman officers kept note books while on campaign and traders were reaching the Baltic coast and would have had indirect information concerning Sweden and the east Baltic lands.