flattening some fairly large diameter copper tubing might be one source. If that won’t work I just happened to go to a garage sale and picked up a serving tray made from copper. I bought it figuring it was perfect material to work with for all sorts of little projects. I think the thickness is petty close to what you were looking for.
heat and quench to make copper soft and heat and cool slowly to make it hard, opposite of steel. perhaps hard and soft tubing has some differences in metalurgy, but I think it’s pretty much true.
any idea what HP you need for the motor? I assume you want one that looks old, but need very little power because it just turns the rotor? a singer sewing machine motor perhaps?