There are very few sources nationwide.
This is a material that is not used much, so the few suppliers can call their own shots, and frequently do.
Besides Atlas, the main copper/copper alloy suppliers are:
each has different areas of specialization- not all may carry silicon bronze sheet. each has different minimums.
none are cheap.
maybe there is another sculptor who has scrap they would sell you- I am not sure where you would find em, though.