Where can i buy a sliver aluminum magnetic board approx 4x3 ft?

Yes, aluminum is not magnetic. A metal must have iron it it (like steel) to be magnetic. So you'll need to find something heavier and paint it the particular color of silver you want. Fortunately there are many great spray paints out there. If you are looking for aluminum because it is lightweight, then I'm afraid you are out of luck. You'll have to decide if the weight or the magnetism is more important to your project.