"I think" (note the qualifier) that the Mossberg 590 series uses the same bolts as the 500 series.
Problem is, I don't think that Mossberg makes a plated bolt in anything but the Marinecote models, and possibly not even in them. The rest are just bright polished steel.
Mossberg will not sell you a bolt unless they install it.
You could just have your bolt plated by a good gun plating company like Ford's or APW/Cogan.