You can't find it for this reason:

I am an idiot and I called it the wrong thing.

You need galvanized steel FLASHING.

https://www.amazon.com/AMERIMAX-PRODUCTS-70006-Galvanized-Flashing/dp/B000BQ8DPW/

6" by 50' for $40 and larger widths are available. Most hardware stores should have this. Bring a magnet with you to make sure you're buying steel and not aluminum or something else.