I found a source for small bits of alu for things like diffuser panels and the engine compartment floor - things that aren't structural:

window installers.

They often have offcuts of small sheets, sometimes anodised (no wider than 2 feet) and are FREE (at least they were for me).

You mean you didnt use ally for the floor?

Yes, but that was included in my kit, as well as some interior panels. Don't see why you couldn't use these suppliers for interior stuff though.