If you want copper strips, couldn't wire from the hardware store
be easily put through the rollers to make flat? This would be
cheap, quick and no shipping charges.
The suggestion below is a good one, but be sure to anneal as you go!
If you really work harden copper, it can become hard enough to
damage the steel rolls in your mill. I still remember the rant on
the subject we got from the shop supervisor in grad school…as he
pointed to a set of slip rolls with nasty grooves in them where
someone had tried to flatten copper round wire to make bus bars.
Here’s an alternative idea: buy some thick foil/thin sheet from
onlinemetals.com and then shear the pieces you need. A little
Profiform shear or the like would make short work of this task. If
you or noone in your circle have a shear, try a high school or
community college in your area. They likely have a foot shear in
their machine shop.