I too had this problem. I removed them, glassbeaded them and painted them with flat black paint...good as new. You could wire wheel them too I suppose.
This type of bolt with a large, captive free spinning washer is not something you will find at a hardware store. An auto body supply shop may have something similar and you could likely buy a box of 25 for what DC would charge for 5 bolts.
As one that has restored many older Mopars, I have in some cases been able to find correct replacements, but have more often had to have the originals replated as the heads are different, markings are different, etc.
FWIW, these bolts were phosphated, which is not the most durable finish. The paint will last much longer. Automotive fasteners in general are not plated with very durable finishes, like the clear zinc on the sill screws.