We have Discount Steel and Metals Supermarket, plus a couple scrap dealers. I've found Metals Supermarket to be a bit more expensive than Discount Steel, but it's closer to our school.
For reference, the 36"x48" 5052 0.063" price at
Discount Steel is $39.57. The 48"x144" is $168. I usually go over there and pick from the cutoff rack as that's often cheaper than getting a newly cut sheet.
Packaging thin metal like that for shipment is very expensive. I agree with others that you should find your local supplier for that sort of material. If you can't figure out who it is, you should call a local machine shop and ask to talk to the person who does their buying -- they'll be able to send you to the right place.