You can get new ones from AMK Products. Check out their website. Their part number for these bolts is B-11709. This car uses about 50 of them. My measurements were: 5/16-18x7/8, Conical Hex Washer (.75 OD), 1/2 AF.
FYI to all....I'm in the process of a complete resto. I have and am measuring and keeping record of every fastener (bolt, nut, washer screw) for the entire car. I will post once the car is complete, shortly here, as a reference. I have based the replacements on the AMK product line.
Thanks,
Tony