I think Smith and Deakin? may sell them. I attached mine with four pins (sliding type) for easy lift off complete removal. It provides great access to the engine bay. And it's light and won't rust.

I'm sure there was an SD1 bonnet with a capri bulge(!) on e-bay recently, but I believe it was fabricated in steel.

I've just had a quick look, but it's not there anymore, hence either item has expired or sold....