I know that this will be discouraging, but the only way that I have found one is to scour every old tractor bone-yard you can find. It took me probably 1,500 miles of driving in Eastern NC to find one lens. The right headlight lens had survived the previous 49 years without mishap until I bought the 1950 Model B. After I bought it, it lasted about 2 days. I dropped it while changing the bulb.
I looked all over until I finally found a lens on some unknown brand of rusted hulk. The guy at the junk yard sold it to me for a dollar, but it cost $100 worth of gas to find it.
Just look around. That light was used on a variety of old tractors. Just make sure your lens is the 5 inch lens. The newer ones are 4 1/2 inches.