Unless you can find someone parting out a 390, you would probably be better off to just buy a used one and take the bolt assembly out of it. You could then sell the remaining parts and probably get all your money back, thereby getting the bolt for free.
BAD303 - I am pretty sure the 300 series bolt is the same. I have an extra but the small lug is peened off it. I am sure a gunsmith could weld and reshape a new lug. Your looking at a pricey item to buy. I ended up buying a gun for parts and parting it out just to get a bolt but later a smith told me he could have fixed it