the power up is between a bunch of buildings where several stormies are shooting them. after you atomize the TIE bombers on ur way to the capital tower, u will see the buildings. this power up is kind of hard to get because buildings are all around u. Plus the power up is slightly lower than the speeder can hover. so, u have to go at an incline. after u do this, go to to the side to avoid crashing into the tall building in front of u. you can also do this if u have any downtime after you shoot down all 6 TIE's attacking the shuttle.
I can't remember where the advanced blaster powerup. but there are websites which will tell you. i'll reply later when i find one.