If no one knows of a supplier of larger sheet, why not pour your
own? Heck, you can melt pewter on a kitchen stove.
You can use an extra heavy duty sheet cake pan, they’re 12 or 13
gauge. Full sheet pans are 18" x 26". Don’t get a non-stick one. The
fiddliest part, I imagine, would be making the pan perfectly level in
preparation for the pour.
Level the pan, preheat it, oil it or spray with silicone release,
melt the pewter in an iron casting ladle, then pour carefully.
I don’t know where you’re at school, but it’s the kind of project
that many silversmithing teachers might enjoy. You could start with a
quarter sheet pan, 9" x 13", to get the hang of it.
Just a notion.