They have a .500 color case hardened target hammer assembly listed at Numrich/Gun Parts Corp listed for your 586 for $65.25 and in stock.
The link to it is
right here. I've bought from them myself and they are one of my trusted vendors for parts.
With that said, you might have to do some fitting to get it to work properly. I've been lucky in the hammer change I did on my original 27-2 when I started having problems and mine was a drop in replacement but not all go that way.