20 mil vinyl

Convex.

 

You could check out Substance. Their stuff is 16 or 21mil depending on the overlaminate. I've gotten a few samples from them and it's pretty good stuff.

 

I would recommend a heavy laminate, but the vinyl itself doesn't need to be that heavy of a mil. Something like Arlon DPF8000/8002 would be a great vinyl for this.

 

I too second Substance Media.

 

This. Substance and Convex are both winners.

 

20 mil is the final thickness after laminating. Convex is the main player in the helmet decal supply business. http://www.convexvinyl.com/helmets/

 

We love convex HighBond (which I believe is 6 mil) with the ProShield 12 mil laminate. Just make sure your cutter is hefty enough to handle it. The Convex chrome is incredible film as well, although a bit high priced.