Julius Caesar's Gallic Wars is here:
http://classics.mit.edu/Caesar/gallic.html
Something useful from Tacitus? here
http://www.sacred-texts.com/cla/tac/ag01010.htm#ag_01_016
Cassius Dio has a little bit on Britain
http://penelope.uchicago.edu/Thayer/E/Roman/Texts/Cassius_Dio/home.html
There's bound to be more somewhere. But that's a start I guess. I'd be interested to read this stuff too so will keep looking.