They're actually pretty easy Kraderic!
Gold Bars are typically:
-In your direct path
-Under a destructible piece of the environment
-(Or) Slightly off the main path
The good news is that in order to fully upgrade your bipedal-tank, you don't need all of the Gold Bars. Meaning that before the end of the game you'll acquire the only Achievement requiring them in the first place. Quite silly, but the developers included way too many gold bars than needed. It's almost like they didn't put much thought into it!