Every plastic shop sells Miata front windshields, you just have to trim off the extra stuff that's not part of the windsheild 
Doesn't take long to do. I used 1/4" when I built ours. The arc of the windshield is minor, you don't need a formed sheet as it'll easy bend to shape.
The hard part is drilling the rivet holes in the windshield frame. It's definitely not the same steel as the rest of the car!
