I have no idea where to get mylar this thin in Europe but I got mine from the US. There is a thread on DIY headphones over on HF running now which has a link to the store.
The stock headphone didn't have mylar to begin with but it was relatively thick. In that time 3-4um was the norm as thinner material wasn't available.
As for curved stators, they are a stupid idea, even the inventor of curved panels has said so. Now staggered panels, that's a whole other thing and quite a good idea.