you don't need spacers for the problem that you described. Spacers change the offset of your wheels and have no bearing on their balance while on the vehicle. You likely need one of two things done:

1. hubcentric rings for all 4 wheels. Go to where you purchased the wheels from or any wheel/tire store and they'll be able to get you a set specific for your car and the wheels you have.

2. your wheels may not have been balanced properly OR they may have thrown a weight following proper balancing. Have them rebalanced to ensure it's not the problem causing your vibration, but I'd hold off on this until you get your hubcentric rings installed.

If it was any issue related to the brake rotors, you'd feel the vibration while braking at speed, not cruising.

 

Wheels aren't balanced.

 

cool thnx.