You can get reproduction WWII chinstraps here:
http://www.atthefront.com/
I've dealt with that company several times and their stuff is top notch. You can get both the web straps that attach to the helmet, and the leather strap that attaches to the liner. Just go to US>headgear>helmet parts, and you'll see a bunch of stuff.
To be accurate to WWII, you'll probably want to remove the little alligator tooth retainer piece on the bails. WWII bails were just rectangular wire rings (either welded on, early war, or on a swivel, later war).